What is MIME type "multipart/related"?

A MIME type is a string that tells browsers and other tools how to handle a particular kind of file.

multipart/related is a MIME type that groups different pieces of content that are meant to work together. It bundles a primary file and its related resources so they can be sent or stored as one unit.

In practice, one part is marked as the main resource. Other parts (like images, style sheets, or scripts) support it. This setup ensures all pieces are available when the file is viewed. For example, web pages saved in MHT or MHTML format use this MIME type to keep content and resources together.

Use Case: Bundling an HTML file and its images, styles, and scripts together.
Use Case: Packaging email messages with embedded media.
Key Fact: It ensures that the main content and its dependent parts are delivered simultaneously.
Key Fact: It uses boundaries to separate parts within the same message.

For more details on how related parts are defined and assembled, see the RFC 2387 on multipart/related.

Associated file extensions

.mht, .mhtml

Usage Examples

HTTP Header

When serving content with this MIME type, set the Content-Type header:


    Content-Type: multipart/related

HTML

In HTML, you can specify the MIME type in various elements:


    <a href="file.dat" type="multipart/related">Download file</a>

Server-side (Node.js)

Setting the Content-Type header in Node.js:


    const http = require('http');
    
    http.createServer((req, res) => {
      res.setHeader('Content-Type', 'multipart/related');
      res.end('Content here');
    }).listen(3000);

Associated file extensions

.mht, .mhtml

FAQ

What is a MIME type?

A MIME (Multipurpose Internet Mail Extensions) type is a standard that indicates the nature and format of a document, file, or assortment of bytes. MIME types are defined and standardized in IETF's RFC 6838.

MIME types are important because they help browsers and servers understand how to process a file. When a browser receives a file from a server, it uses the MIME type to determine how to display or handle the content, whether it's an image to display, a PDF to open in a viewer, or a video to play.

MIME types consist of a type and a subtype, separated by a slash (e.g., text/html, image/jpeg, application/pdf). Some MIME types also include optional parameters.

How do I find the MIME type for a file?

You can check the file extension or use a file identification tool such as file --mime-type on the command line. Many programming languages also provide libraries to detect MIME types.

Why are multiple MIME types listed for one extension?

Different applications and historical conventions may use alternative MIME identifiers for the same kind of file. Showing them all helps ensure compatibility across systems.