The global function mimeTypeMatch() is completly unused, see codesearch: https://codesearch.wmcloud.org/search/?q=mimeTypeMatch&i=nope&files=&excludeFiles=&repos=. It also doesn't fit our Coding conventions, see https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP#Functions.
I would propose that it can be removed as it does no longer seem to be necessary. The only usage was removed with rMW269e7e1d4480: Drop global function wfNegotiateType, deprecated in 1.36.