Here is the link:
https://github.com/user-none/Sigil/b...PFResource.cpp
At the very bottom of this file you will find a mapping table.
The plugin uses its own version of this and they may not be fully in sync but they are close. In a plugin launcher code there is a easy way to get the media-type if you provide the basename of the file with extension. It also provides the media-type for every font file. So the plugin should already easily be able to supply you with the media-type for any file existing in the epub that has an entry in the manifest.
KevinH