]> Adobe

masinter@adobe.com http://larry.masinter.net

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.

There is a GitHub repository for this draft at https://github.com/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, apps-discuss@ietf.org. Please raise issues in the tracker, or send to the apps-discuss list.

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 and subsequently incorporated into and , 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.

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.

The media-type multipart/form-data generally follows the model of multipart MIME data streams as described in Section 5.1, although common implementations vary, as noted in this document. A multipart/form-data body contains a series of parts. Each part MUST contain a Content-Disposition header 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 ). 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.

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.,

multipart/form-data;boundary="-AaB03x"

Note that many implementations do not quote the boundary parameter, e.g.,

multipart/form-data;boundary="-AaB03x"

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 ; this is generally the way that a "file:" URI would be encoded.

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.

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/octet-stream.

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:

--AaB03x content-disposition: form-data; name="field1" content-type: text/plain;charset=UTF-8 content-transfer-encoding: quoted-printable Joe owes =E2=82=AC100. --AaB03x

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 ). 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:

--AaB03x content-disposition: form-data; name="_charset_" content-type: text/plain iso8859-1 --AaB03x-- content-type: text/plain content-disposition: form-data; name="field1" abcdefghijklmnopqrstuvwxyz AaB03x--

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 for more details.) When transferred via HTTP, Content-Transfer-Encoding the form-data values SHOULD NOT be used.

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.

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.

MIME headers in multipart/form-data are required to consist only of 7-bit data in the US-ASCII character set. While suggested that non-ASCII field names should be encoded according to the method in if they contain characters outside of US-ASCII, practice varies. This specification makes three sets of recommendations for three different states of workflow.

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 users.

Some applications of this specification will supply a character encoding to be used for creation of the multipart/form-data result. In particular, uses: the value of an accept-charset attribute of the <form> element, if there is one, the character encoding of the document containing the form, if it is US-ASCII compatible, 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 . 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.

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 and used the "encoded-word" method of encoding non-ASCII values:

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

Others have been known to follow , 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 ).

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

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

name=Xavier+Xantico&verdict=Yes&colour=Blue&happy=sad&Utf%F6r=Send

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.

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.)

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

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 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.

multipart form-data boundary none For use in transports that restrict the encoding to 7BIT or 8BIT, each part is encoded separately. 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 of this document.

&rfc.2046; &rfc.2047; &rfc.2231; &rfc.1806; &rfc.2183; &rfc.1867; &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 . 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 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 documented. 'boundary' is a required parameter in Content-Type. 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 Introduction.

There are numerous alternative ways in which form data can be encoded; many are listed in 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 significant. More problematic is the ambiguity introduced because implementations did not follow 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.