Magick: Skip one-letter formats

ImageMagick reads .R, .G, .B, .C, .M., .Y, .A, .O, .. as raw image
channel files. I don't feel that those formats should be in pqiv: Image
loading scans folders for files matching the extensions and
optimistically adds all of them to the list of images. One-letter
extensions can mean _anything_, IMHO, so this adds lots of non-image
files (.C for C, .M for matlab, etc.) which have to be removed when
ImageMagick failed to load them.