Network Working Group                                        L. Masinter
Internet-Draft                                                     Adobe
Obsoletes: 2388 (if approved)                             April 14, 2014
Intended status: Standards Track
Expires: October 16, 2014

            Returning Values from Forms: multipart/form-data


   This specification (re)defines the multipart/form-data Internet Media
   Type, which can be used by a wide variety of applications and
   transported by a wide variety of protocols as a way of returning a
   set of values as the result of a user filling out a form.  It
   replaces RFC 2388.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 16, 2014.

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   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  NOTE  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Advise for Forms and Form Processing  . . . . . . . . . . . .   3
   4.  Definition of multipart/form-data . . . . . . . . . . . . . .   3
     4.1.  Boundary  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  filename attribute  . . . . . . . . . . . . . . . . . . .   4
     4.3.  Multiple files for one form field . . . . . . . . . . . .   4
     4.4.  Content-Type  . . . . . . . . . . . . . . . . . . . . . .   5
     4.5.  The text/plain charset parameter for text fields  . . . .   5
     4.6.  The _charset_ field . . . . . . . . . . . . . . . . . . .   5
     4.7.  Content-Transfer-Encoding . . . . . . . . . . . . . . . .   6
     4.8.  Other Content- headers  . . . . . . . . . . . . . . . . .   6
   5.  URL encoding non-ASCII values . . . . . . . . . . . . . . . .   6
   6.  Operability considerations  . . . . . . . . . . . . . . . . .   6
     6.1.  Non-ASCII field names and values  . . . . . . . . . . . .   6
       6.1.1.  Avoid creating forms with non-ASCII field names . . .   6
       6.1.2.  Interpreting forms and creating form-data . . . . . .   7
       6.1.3.  Parsing and interpreting form data  . . . . . . . . .   7
     6.2.  Ordered fields and duplicated field names . . . . . . . .   7
     6.3.  Interoperability with web applications  . . . . . . . . .   8
     6.4.  Correlating form data with the original form  . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Media type registration for multipart/form-data . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Changes from RFC 2388  . . . . . . . . . . . . . . .  10
   Appendix B.  Alternatives . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  NOTE

   There is a GitHub repository for this draft at
   masinter/multipart-form-data along with an issue tracker.  This
   specification has been proposed as a work item of the APPSAWG
   Applications Area working group,  Please raise
   issues in the tracker, or send to the apps-discuss list.

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2.  Introduction

   In many applications, it is possible for a user to be presented with
   a form.  The user will fill out the form, including information that
   is typed, generated by user input, or included from files that the
   user has selected.  When the form is filled out, the data from the
   form is sent from the user to the receiving application.

   The definition of "multipart/form-data" is derived from one of those
   applications, originally set out in [RFC1867] and subsequently
   incorporated into [HTML3.2] and [HTML4], where forms are expressed in
   HTML, and in which the form values are sent via HTTP or electronic
   mail.  This representation is widely implemented in numerous web
   browsers and web servers.

   However, multipart/form-data can be used for forms that are presented
   using representations other than HTML (spreadsheets, PDF, etc.), and
   for transport using means other than electronic mail or HTTP.  This
   document defines the representation of form values independently of
   the application for which it is used.

3.  Advise for Forms and Form Processing

   The representation and interpretation of forms and the nature of form
   processing is not specified by this document.  However, for forms and
   form-processing that result in generation of multipart/form-data,
   some suggestions are included.

   In a form, there are a generally series of fields, where each field
   is expected to be supplied with a value, e.g., by the user who fills
   out the form.  Each field has a name.  After a form has been filled
   out, and the form's data is to be 'submitted', the form processing
   results in a set of values for each field-- the "form data".

   In forms that work with multipart/form-data, field names are Unicode
   strings, but restricting field names to ASCII avoids some
   interoperability issues.  Within a given form, insuring field names
   are unique is helpful.  Some fields may have default values or
   presupplied values in the form itself.  Fields with presupplied
   values might be hidden or invisible.

4.  Definition of multipart/form-data

   The media-type "multipart/form-data" generally follows the model of
   multipart MIME data streams as described in [RFC2046] Section 5.1,
   although common implementations vary, as noted in this document.

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   A "multipart/form-data" body contains a series of parts.  Each part
   MUST contain a "Content-Disposition" header [RFC1806] [RFC2183] where
   the disposition type is "form-data", and where the disposition
   contains an (additional) parameter of "name"; the value of the
   parameter is the original field name from the form (encoded, see
   Section 6.1).  For example, a part might contain a header:

           Content-Disposition: form-data; name="user"

   with the value corresponding to the entry of the "user" field.

4.1.  Boundary

   As with other multipart types, the parts are delimited with a
   boundary, selected such that it does not occur in any of the data.
   Each field of the form is sent, in the order defined by the sending
   application and form, as a part of the multipart stream.  The
   boundary is supplied as a "boundary" parameter to the multipart/form-
   data type, e.g.,


   Note that many implementations do not quote the boundary parameter,


4.2.  filename attribute

   For form data that represents the content of a local file, a name for
   the file SHOULD be supplied as well, by using a ?filename? parameter
   of the ?Content-Disposition? header.  (The "SHOULD" is to allow file
   uploads that result from drag-and-drop in systems where the file name
   is meaningless or private, where the form data content is streamed
   directly from a device, or where the file name is not user visible
   and would be unrecognized.)

   For compatibility with other multipart types, the value of the
   "filename" parameter MUST be restricted to US-ASCII.  File names
   normally visible to users which contain non-ASCII characters SHOULD
   be encoded using the URL-encoding method in Section 5; this is
   generally the way that a "file:" URI would be encoded.

4.3.  Multiple files for one form field

   If the value of a form field is a set of files rather than a single
   file, that value MUST be transmitted by supplying each in a separate
   part, but all with the same "name" parameter.

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4.4.  Content-Type

   Each part has an (optional) "Content-Type", which defaults to "text/
   plain".  If the contents of a file are to be sent, the file data is
   labeled with an appropriate media type, if known, or "application/

4.5.  The text/plain charset parameter for text fields

   In the case where a field value is text, the charset parameter for
   the "text/plain" Content-Type may be used to indicate the character
   encoding used in that part.  For example, a form with a text field in
   which a user typed "Joe owes <eu>100" where <eu> is the Euro symbol
   might have form data returned as:

         content-disposition: form-data; name="field1"
         content-type: text/plain;charset=UTF-8
         content-transfer-encoding: quoted-printable

         Joe owes =E2=82=AC100.

4.6.  The _charset_ field

   Some form processing applications have the convention that the value
   of a form entry with entry name "_charset_" and type "hidden" is
   automatically set; the value is then used as the default charset of
   text field values (see form-charset in Section 6.1.2).  In this case,
   the value of the default charset of each text/plain part without a
   charset parameter is set to the supplied value.  For example:

         content-disposition: form-data; name="_charset_"
         content-type: text/plain

         content-type: text/plain
         content-disposition: form-data; name="field1"


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4.7.  Content-Transfer-Encoding

   When used in transports which do not allow arbitrary binary data,
   each part that cannot be represented within the transport SHOULD be
   encoded and the "Content-Transfer-Encoding" header supplied in that
   part.  For example, some email transports use a 7BIT encoding.  (See
   section 5 of [RFC2046] for more details.)  When transferred via HTTP,
   Content-Transfer-Encoding the form-data values SHOULD NOT be used.

4.8.  Other Content- headers

   The "multipart/form-data" media type does not support any MIME
   headers in the parts other than Content-Type, Content-Disposition,
   and (when appropriate), Content-Transfer-Encoding.

5.  URL encoding non-ASCII values

   Within this specification, "URL-encoding" is used for representing
   non-ASCII characters or those that are not allowed in a context.  The
   encoding is created replacing each non-ASCII or disallowed character
   with a sequence, where each byte of the UTF-8 encoding of the
   character is represented by a percent-sign (%) followed by the (lower
   case) hexadecimal of that byte.

6.  Operability considerations

6.1.  Non-ASCII field names and values

   MIME headers in multipart/form-data are required to consist only of
   7-bit data in the US-ASCII character set.  While [RFC2388] suggested
   that non-ASCII field names should be encoded according to the method
   in [RFC2047] if they contain characters outside of US-ASCII, practice

   This specification makes three sets of recommendations for three
   different states of workflow.

6.1.1.  Avoid creating forms with non-ASCII field names

   For broadest interoperability with existing deployed software, those
   creating forms SHOULD avoid non-ASCII field names.  This should not
   be a burden, because in general the field names are not visible to

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6.1.2.  Interpreting forms and creating form-data

   Some applications of this specification will supply a character
   encoding to be used for creation of the multipart/form-data result.
   In particular, [HTML5] uses:

   o  the value of an accept-charset attribute of the <form> element, if
      there is one,

   o  the character encoding of the document containing the form, if it
      is US-ASCII compatible,

   o  otherwise UTF-8.

   Call this the form-charset.  Any field name or file name which is not
   in US-ASCII MUST be encoded by the URL-encoding method in Section 5.

   multipart/form-data parts which do not have a Content-Type header and
   which are not the result of supplying a local file MUST be
   transformed by the same algorithm.

6.1.3.  Parsing and interpreting form data

   While this specification provides guidance for creation of multipart/
   form-data, interpreters of multipart/form-data should be aware of the
   variety of implementations.  Currently, deployed browsers differ as
   to how they encode multipart/form-data.  For this reason the matching
   of form elements to form-data parts may rely on a fuzzier match.  In
   particular, some form-data generators might have followed the
   previous advice of [RFC2388] and used the [RFC2047] "encoded-word"
   method of encoding non-ASCII values:

        encoded-word = "=?" charset "?" encoding "?" encoded-text "?="

   Others have been known to follow [RFC2231], to send unencoded UTF-8,
   or even unencoded strings in the form-charset.

   Generally, interpreting "multipart/form-data" (even from conforming
   generators) may require knowing the charset used in form encoding, in
   cases where the _charset_ field value or a charset parameter of a
   text/plain Content-Type header is not supplied.  (Note ).

6.2.  Ordered fields and duplicated field names

   Form processors given forms with a well-defined ordering SHOULD send
   back results in the order received and preserve duplicate field
   names, in order.  Intermediaries MUST NOT reorder the results.  (Note

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   that there are some forms which do not define a natural order of

6.3.  Interoperability with web applications

   Many web applications use the "application/x-url-encoded" method for
   returning data from forms.  This format is quite compact, e.g.:


   However, there is no opportunity to label the enclosed data with
   content type, apply a charset, or use other encoding mechanisms.

   Many form-interpreting programs (primarily web browsers) now
   implement and generate multipart/form-data, but an existing
   application might need to optionally support both the application/x
   -url-encoded format as well.

6.4.  Correlating form data with the original form

   This specification provides no specific mechanism by which multipart/
   form-data can be associated with the form that caused it to be
   transmitted.  This separation is intentional; many different forms
   might be used for transmitting the same data.  In practice,
   applications may supply a specific form processing resource (in HTML,
   the ACTION attribute in a FORM tag) for each different form.
   Alternatively, data about the form might be encoded in a "hidden
   field" (a field which is part of the form but which has a fixed value
   to be transmitted back to the form-data processor.)

7.  IANA Considerations

   IANA please update the registration of multipart/form-data to point
   to this document.

8.  Security Considerations

   It is important when interpreting the filename of the Content-
   Disposition header to not overwrite files in the recipient's file
   space inadvertently.

   User applications that request form information from users must be
   careful not to cause a user to send information to the requestor or a
   third party unwillingly or unwittingly.  For example, a form might
   request 'spam' information to be sent to an unintended third party,
   or private information to be sent to someone that the user might not
   actually intend.  While this is primarily an issue for the
   representation and interpretation of forms themselves (rather than

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   the data representation of the form data), the transportation of
   private information must be done in a way that does not expose it to
   unwanted prying.

   With the introduction of form-data that can reasonably send back the
   content of files from a user's file space, the possibility arises
   that a user might be sent an automated script that fills out a form
   and then sends one of the user's local files to another address.
   Thus, additional caution is required when executing automated
   scripting where form-data might include a user's files.

9.  Media type registration for multipart/form-data

   Media Type name:  multipart

   Media subtype name:  form-data

   Required parameters:  boundary

   Optional parameters:  none

   Encoding considerations:  For use in transports that restrict the
      encoding to 7BIT or 8BIT, each part is encoded separately.

   Security considerations:  Applications which receive forms and
      process them must be careful not to supply data back to the
      requesting form processing site that was not intended to be sent
      by the recipient.  This is a consideration for any application
      that generates a multipart/form-data.  See Section 8 of this

10.  References

10.1.  Normative References

   [RFC1806]  Troost, R. and S. Dorner, "Communicating Presentation
              Information in Internet Messages: The Content-Disposition
              Header", RFC 1806, June 1995.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

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   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

10.2.  Informative References

   [HTML3.2]  Raggett, D., "HTML 3.2 Reference Specification", World
              Wide Web Consortium Recommendation REC-html32-19970114,
              January 1997, <>.

   [HTML4]    Raggett, D., Hors, A., and I. Jacobs, "HTML 4.0
              Recommendation", World Wide Web Consortium REC-
              html40-971218, December 1997,

   [HTML5]    Berjon, R., Faulkner, S., Leithead, T., Navara, E.,
              O'Connor, E., and S. Pfeiffer, "HTML5", September 2013,

   [RFC1867]  Nebel, E. and L. Masinter, "Form-based File Upload in
              HTML", RFC 1867, November 1995.

   [RFC2388]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 2388, August 1998.

Appendix A.  Changes from RFC 2388

   The handling of non-ASCII field names changed-- no longer
   recommending the RFC 2047 method, instead requiring URL encoding; few
   implemented the =?charset:string?= method of [RFC2047].

   The handling of multiple files submitted as the result of a single
   form field (e.g., HTML's <input type=file multiple> element) results
   in each file having its own top level part with the same name
   parameter; the method of using a nested "multipart/mixed" from
   [RFC2388] is no longer recommended for creators, and there are no
   known implementations of senders.

   The _charset_ convention and use of an explicit form-data charset is

   'boundary' is a required parameter in Content-Type.

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   The relationship of the ordering of fields within a form and the
   ordering of returned values within multipart/form-data was not
   defined before, nor was the handling of the case where a form has
   multiple fields with the same name.

   Editorial: Removed obsolete discussion of alternatives in appendix.
   Update references.  Move outline of form processing into

Appendix B.  Alternatives

   There are numerous alternative ways in which form data can be
   encoded; many are listed in [RFC2388] section 5.2.  The multipart/
   form-data encoding is verbose, especially if there are many fields
   with short values.  In most use cases, this overhead isn't

   More problematic is the ambiguity introduced because implementations
   did not follow [RFC2388] because it used "may" instead of "MUST" when
   specifying encoding of field names, and for other unknown reasons, so
   now, parsers need to be more complex for fuzzy matching against the
   possible outputs of various encoding methods.

Author's Address

   Larry Masinter


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