Also ".sun" is a possible (and common) file name extension for Sun
Raster images, according to various sources and samples I found (these
samples with .sun extension also opened fine in GIMP, so it's not just a
subvariant which we may not handle or something of the sort). This one
is not so important though as we also register magic bytes for detection
(which is the proper way), but it can still be useful, mostly for
exporting (as we will direct to the SunRaster plug-in if someone tried
to export a file with .sun extension, since no other file format uses
this extension AFAICS).

There is no functional change, I just had a look at this plug-in while
handling !428 and realized this format was not present in the MimeType
list (which is used to generate the desktop file, in order to have
proper mime types, not detection based on extension only, unlike
Windows in !428).