I have a number of albums I've purchased directly from artist websites that include the liner notes in pdf format. I also have other downloads (several from the Avant-Garde Project for instance) that have the original notes and additional information as text files. On Rhythmbox startup, these end up as "Import Errors" with "The MIME type of the file could not be identified" as the error.

I understand about music files without a codec to support them generating these errors, but having liner notes, album cover art, etc. included with music files should be pretty common. All of these files have MIME types recognized within Gnome (i.e. pdfs have thumbnail images, text files have text previews, etc.). They should not generate errors, especially when the only context menu option for these "errors" is "Move to Trash"!