MIME Type

multipart/form-data

Form

MIME type used when submitting forms that include files or mixed fields.

MIME type reference, HTTP example, browser usage, common mistakes, and related content.

What is the multipart/form-data MIME type?

The MIME type multipart/form-data is used to tell browsers, APIs, and servers how a file or response body should be interpreted.

MIME stands for Multipurpose Internet Mail Extensions, and MIME types are now a standard part of HTTP responses and web content delivery.

When a browser or client receives a response with multipart/form-data, it uses that information to decide how the content should be processed, rendered, downloaded, or executed.

Example

Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryXYZ

HTTP example

HTTP/1.1 200 OK
Content-Type: multipart/form-data
Content-Length: 1256

Common use cases

File uploads
HTML form submissions
Mixed text and binary payloads

Common mistakes

Using the wrong MIME type for the file being served
Returning text/plain instead of multipart/form-data
Forgetting required parameters like charset when relevant
Using a deprecated MIME type in older server configurations
Serving assets with a mismatched Content-Type header, causing browser parsing issues

How browsers use it

Browsers use the Content-Type response header to decide how a response should be handled. For example, HTML is rendered as a page, CSS is parsed as styles, JavaScript is executed as script, and images are displayed visually. If the MIME type is incorrect, the browser may refuse to load the file correctly or may treat it as plain text or a download instead.

Developer note

Critical for file uploads. If your backend upload route is failing, this MIME type is often involved.

Related MIME types

application/x-www-form-urlencoded application/octet-stream image/png