Handle non-uppercase tag names correctly.

Some FLAC files -- even those produced by "reference libFLAC 1.3.0 20130526" as used by [fre:ac](http://www.freac.org/) program -- use "Artist" tag which was not recognized as all tags were expected to be in upper case.

Fix this by simply forcing the tags to be upper-case before looking them up.
Don't mix up a picture's type with its description.
Merge remote-tracking branch 'vadz/patch-1'