fetchart: Get all cover art types from the Cover Art Archive

[srussel]

I would like to be able to get the back cover and booklet from the coverart archive. It would be great fetchart could retrieve all art types from the covertart archive and use and allow the user to set a config option to select which types will be retrieved.

[sampsyo]

Sounds desirable, but this one will be more complicated than it might seem at first: the beets data model is only set up currently to track a single art file per album. This would be a great use case for a more generic “attachments” mechanism.

[srussel]

Perhaps as a first step fetchart could just download the files to the album directory. It does not bother me if beets doesn't know about the additional files. I'd just like to have the files available to refer to when I am away and don't have access to the CDs.

[msdos]

@srussel about attachments mechanism, check some discussion in #111