mimetypes are parsed from the file /etc/mime.types cat /etc/mime.types | grep javascript application/javascript js application/x-javascript js actual: mimetypes.guess_type("x.js") == "application/x-javascript" -> deprecated mimetype expected: mimetypes.guess_type("x.js") == "application/javascript" spoiler: here, the "x-" part is deprecated. mimetypes.guess_type returns the deprecated mimetype because python returns the last entry in the /etc/mime.types file which is sorted alphabetically proposed solution: use a smarter conflict-resolution when parsing /etc/mime.types. when two entries are found for one file-extension, avoid using a deprecated mimetype. rfc4329 lists 16 items for "unregistered media type" these could be hard-coded as set-of-strings, or as regex. related bug report https://bugs.python.org/issue46035 mimetypes.guess_type https://docs.python.org/3/library/mimetypes.html#mimetypes.guess_type unregistered media type https://datatracker.ietf.org/doc/html/rfc4329#section-3