[imagedump] Add "list MIME type" (-m) option
Allows listing the supported mime types
More useful than the image format names:
$ bin/imagedump -m
MIME types:
application/pdf
application/x-krita
application/x-navi-animation
image/bmp
image/gif
image/jpeg
image/openraster
image/png
image/svg+xml
image/svg+xml-compressed
image/tiff
...
Edited by Kai Uwe Broulik