Bugfixing for two of the extensions

Fixing bugs I found while running the extension tests from the seperate
extension repository
https://github.com/gpodder/gpodder-hook-scripts/tree/master-core
Add option to auto embed coverart image (bug 1742)

Refactored the code to be able to add filetype specific logic.
Until now we used only the mutagen abstraction to implement one
code for all file types, but this don't work for special needs
(e.g: coverart manipulation)