That message appears each time when I import the music or on each start of the Rhythmbox (which is really annoying). It's stumble on the files kind of DOC, INFO, CUE, JPEG etc... which usually people keep with albums together, and offer codec search. And you have no chance to ignore it, even if you ignore it  literally and hide dialog under the window (because it's really bizarre to click several hundred times each time on start). But most horrible, that blocks importing, so, if you add a new music, you forced to do that procedure. 

That will be good to add "Ignore" and "Ignore All" button in the dialog. And maybe add some setting to do that automatically. This is, IMHO, most annoying thing in the Rhythmbox at this moment. Everything other is practically excellent.