Let imagemagick detect file format (bug #2332)

Let imagemagick detect file format (bug #2332)

Imagemagick detects the inut file format based on contents. Therefore it does
not make sense that we prefix the to be converted file name with the extension
(assuming that the file extension matches the imagemagick format name). This
breaks formats where the extension used by LyX does not match the imagemagick
format name.