fix album art caching for files without artist/album tags

fix album art caching for files without artist/album tags

Closes: #5298
If a file has no metadata, but has a download URL, we download the image
and cache it using only the URL.
Thus files with attachments and empty tags always used the same image,
because we matched the image to the URL "attachment://X"

Add the title (or file name as fallback) to the cache name when using
attachments for files without artist/album meta.