Explainer1 min readUpdated July 1, 2026

What is a MIME type?

Short answer

A MIME type (or media type) is a label like text/html or image/png that tells software the format of a file or response. It has a type and subtype separated by a slash, and browsers use it to decide how to handle content.

What a MIME type does

A MIME type (now officially a media type) identifies a file's format so software knows how to process it. On the web, servers send it in the Content-Type header, and the browser decides whether to render, play, or download the content. See what is an HTTP header.

How they are structured

text/html; charset=utf-8
|   |      |
type subtype parameter

type/subtype, optionally with parameters

text/html — a web page
application/json — a JSON payload
image/png — a PNG image
application/pdf — a PDF document
application/octet-stream — unknown binary, usually downloaded

Try it: HTTP Header InspectorInspect the Content-Type and other headers a URL returns.Open tool

The wrong MIME type breaks thingsIf a server sends the wrong Content-Type, a browser may download a page instead of rendering it, or refuse a script. Security headers like X-Content-Type-Options: nosniff make the declared type authoritative.

Where MIME types appear

The HTTP Content-Type and Accept headers
Email attachments (MIME began with email)
HTML <input accept> and <source type> attributes
File upload validation

References

Questions

What is the difference between a MIME type and a file extension?

A file extension is a hint in the file name (.png), while a MIME type is a declared label sent with the data (image/png). Servers and browsers trust the MIME type over the extension, which is why the Content-Type header matters.

What MIME type is JSON?

application/json. Sending JSON with the correct type lets clients parse it automatically and is required by many APIs.

Does this send my data anywhere?

No. Utilumo's developer tools parse and transform input inside the browser tab. Nothing is uploaded, stored, or logged.

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