Fixes #2504
Fixes #2504

PIL Image.save() requires a string parameter [1] while under python3 we call it with bytes.
This leads to wrong format detection (b'.png' isn't a key in supported formats list).

[1] https://pillow.readthedocs.io/en/latest/reference/Image.html#PIL.Image.Image.save

ibmibmibm/beets,shamangeorge/beets,sampsyo/beets,SusannaMaria/beets,shamangeorge/beets,SusannaMaria/beets,SusannaMaria/beets,ibmibmibm/beets,jackwilsdon/beets,shamangeorge/beets,ibmibmibm/beets,beetbox/beets,beetbox/beets,jackwilsdon/beets,sampsyo/beets,beetbox/beets,shamangeorge/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,SusannaMaria/beets,beetbox/beets,jackwilsdon/beets,sampsyo/beets