MIME type
Also called: Content type, Media type
A label like image/jpeg or application/pdf that says what kind of data a file or response contains.
Definition
Browsers and servers use MIME types to decide how to handle files. Wrong MIME types cause downloads instead of previews, or rejected uploads.
In practice
An upload form accepts application/pdf, but the server returns PDFs as text/plain, so they display as gibberish.