Can I exclude files from Rhythmbox library by filetype?

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http://askubuntu.com – I use Band-in-a-box ("BIAB") to create backing tracks to practice my guitar-playing, and keep the source files (filetype .MGU) created by BIAB in the same folder as the MP3 files derived from them. Because I share this folder with colleagues via Dropbox, I'm not in a position to move the .MGU files elsewhere. Every time I start Rhythmbox ("RB") it checks my music folders, and reports "Import Errors" on all the .MGU files. RB apparently ignores a number of unplayable filetypes in music folders - is there a way of adding .MGU to this group? I know I can just ignore the Import Errors, but one (HowTos)