There's a lot in Moodle (and other websites) that is aimed at stopping admins and other users from doing something stupid - all kinds of input gets cleaned, or prevented in the first place.
I (personally) don't see any benefit in removing such conditions and then dealing in the forums with the 'my logo wont display - its a RAW/Base64/pdf/ico file' or any other kind of file that the software just isn't intended to handle, or displays slightly differently.
Of course you are always free to comment out that line yourself anyway, but my personal feeling would be to add the alternative acceptable formats in to the existing control rather than remove it altogether.