RFC 2388 - Returning Values from Forms: multipart/form-data

1. Abstract

This specification defines an Internet Media Type, multipart/form-data, 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.¶

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 [HTML40], 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, Portable Document Format, etc), and for transport using other means than electronic mail or HTTP. This document defines the representation of form values independently of the application for which it is used.¶

3. Definition of multipart/form-data

The media-type multipart/form-data follows the rules of all multipart MIME data streams as outlined in [RFC2046]. In forms, there are a series of fields to be supplied by the user who fills out the form. Each field has a name. Within a given form, the names are unique.¶

"multipart/form-data" contains a series of parts. Each part is expected to contain a content-disposition header [RFC2183] where the disposition type is "form-data", and where the disposition contains an (additional) parameter of "name", where the value of that parameter is the original field name in the form. 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.¶

Field names originally in non-ASCII character sets may be encoded within the value of the "name" parameter using the standard method described in RFC 2047.¶

As with all multipart MIME types, each part has an optional "Content-Type", which defaults to text/plain. If the contents of a file are returned via filling out a form, then the file input is identified as the appropriate media type, if known, or "application/octet-stream". If multiple files are to be returned as the result of a single form entry, they should be represented as a "multipart/mixed" part embedded within the "multipart/form-data".¶

Each part may be encoded and the "content-transfer-encoding" header supplied if the value of that part does not conform to the default encoding.¶

4. Use of multipart/form-data

4.1. Boundary

As with other multipart types, a boundary is selected that does not occur in any of the data. Each field of the form is sent, in the order defined by the sending appliction and form, as a part of the multipart stream. Each part identifies the INPUT name within the original form. Each part should be labelled with an appropriate content-type if the media type is known (e.g., inferred from the file extension or operating system typing information) or as "application/octet-stream".¶

4.2. Sets of files

If the value of a form field is a set of files rather than a single file, that value can be transferred together using the "multipart/mixed" format.¶

4.3. Encoding

While the HTTP protocol can transport arbitrary binary data, the default for mail transport is the 7BIT encoding. The value supplied for a part may need to be encoded and the "content-transfer-encoding" header supplied if the value does not conform to the default encoding. [See section 5 of RFC 2046 for more details.]¶

4.4. Other attributes

Forms may request file inputs from the user; the form software may include the file name and other file attributes, as specified in [RFC2184].¶

The original local file name may be supplied as well, either as a "filename" parameter either of the "content-disposition: form-data" header or, in the case of multiple files, in a "content-disposition: file" header of the subpart. The sending application MAY supply a file name; if the file name of the sender's operating system is not in US-ASCII, the file name might be approximated, or encoded using the method of RFC 2231.¶

This is a convenience for those cases where the files supplied by the form might contain references to each other, e.g., a TeX file and its .sty auxiliary style description.¶

4.5. Charset of text in form data

Each part of a multipart/form-data is supposed to have a content-type. In the case where a field element is text, the charset parameter for the text indicates the character encoding used.¶

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=windows-1250
    content-transfer-encoding: quoted-printable

    Joe owes =80100.
    --AaB03x

5. Operability considerations

5.1. Compression, encryption

Some of the data in forms may be compressed or encrypted, using other MIME mechanisms. This is a function of the application that is generating the form-data.¶

5.2. Other data encodings rather than multipart

Various people have suggested using new mime top-level type "aggregate", e.g., aggregate/mixed or a content-transfer-encoding of "packet" to express indeterminate-length binary data, rather than relying on the multipart-style boundaries. While this would be useful, the "multipart" mechanisms are well established, simple to implement on both the sending client and receiving server, and as efficient as other methods of dealing with multiple combinations of binary data.¶

The multipart/form-data encoding has a high overhead and performance impact if there are many fields with short values. However, in practice, for the forms in use, for example, in HTML, the average overhead is not significant.¶

5.3. Remote files with third-party transfer

In some scenarios, the user operating the form software might want to specify a URL for remote data rather than a local file. In this case, is there a way to allow the browser to send to the client a pointer to the external data rather than the entire contents? This capability could be implemented, for example, by having the client send to the server data of type "message/external-body" with "access-type" set to, say, "uri", and the URL of the remote data in the body of the message.¶

5.4. Non-ASCII field names

Note that MIME headers are generally required to consist only of 7-bit data in the US-ASCII character set. Hence field names should be encoded according to the method in RFC 2047 if they contain characters outside of that set.¶

5.5. Ordered fields and duplicated field names

The relationship of the ordering of fields within a form and the ordering of returned values within "multipart/form-data" is not defined by this specification, nor is the handling of the case where a form has multiple fields with the same name. While HTML-based forms may send back results in the order received, and intermediaries should not reorder the results, there are some systems which might not define a natural order for form fields.¶

5.6. 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.:¶

   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 (primarly 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.¶

5.7. 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.)¶

6. Security Considerations

The data format described in this document introduces no new security considerations outside of those introduced by the protocols that use it and of the component elements. It is important when interpreting content-disposition to not overwrite files in the recipients address 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 result of form transmission, 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 user's file space, the possibility that a user might be sent an automated script that fills out a form and then sends the user's local file to another address arises. Thus, additional caution is required when executing automated scripting where form-data might include user's files.¶

Appendix A. Media type registration for multipart/form-data

Media Type name:: multipart
Media subtype name:: form-data
Required parameters:: none
Optional parameters:: none
Encoding considerations:: No additional considerations other than as for other multipart types.
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.
The multipart/form-data type introduces no new security considerations for recipients beyond what might occur with any of the enclosed parts.

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Returning Values from Forms: multipart/form-data

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