MIME type lookup (file extension to content type)

How does a server choose a file's MIME type?

A MIME type, or media type, is two names separated by a slash: a top-level type such as image, text or application, then a subtype, as in image/png. A server sends it in the Content-Type header, and MDN points out that browsers decide how to handle a response from that header, not from the file extension, so the server has to send the right one.

IANA keeps the official list. RFC 6838 sets the rules: types of general interest go in the standards tree (image/png), product-specific ones in the vendor tree with a vnd. prefix (application/vnd.ms-excel), and the x. tree is for private use and strongly discouraged. Older names that start with x-, such as application/x-tar, were never registered, yet MDN still lists several of them as the usual type for their files.

Two defaults cover what isn't known: MDN says text/plain is the default for textual files and application/octet-stream for everything else, which browsers download rather than display. Text types take a charset parameter, as in text/html; charset=utf-8, while JSON has none, because RFC 8259 requires UTF-8 when JSON is exchanged between systems.

This lookup holds a curated list of 73 common types, not all of IANA's registry. Each was checked against IANA's registry (last updated 2026-10-09) and against MDN's common types table on 2026-10-20; the 9 that are common but not registered with IANA are marked as such.

type/subtype[; parameter=value]
image/png
text/html; charset=utf-8
application/vnd.openxmlformats-officedocument.wordprocessingml.document
unknown binary → application/octet-stream