Use tab view for "File info" and "Artwork"

[humdingerb]

You could put "File info" and "Artwork" in a tab view and save which one was shown on the last quit. This would remove the need to have an "Appearance" menu, if you agree that the selection colour is not a really useful "micro-customization"... :)

[m199]

We should improve the whole FileInfoBox

• Add a Drag and Drop Area for artwork
• Context menu with delete/change artwork
• deactivate the Tab "Artwork" in the Properties Window.
• Hideable

[humdingerb]

You've already closed #11, so I put it here.

You may want to consider that the FileInfo/Artwork area in the main window feels more like a display of info, not for editing info. IMO, that still fits better into the Properties window.
You could put the buttons of the current Artwork tab into the Details tab, under the Artist/Album info at the top.
Or - though less discoverable - have a context menu on the artwork image:

Select cover for song…
Select cover for album…

Remove cover from song
Remove cover from album

Maybe show this context menu even on a left-click, though in this day and age, I think most user will try to right-click an object they want to manipulate.