In GIOGetMimeType(), removed sniff_data, add comments (improvements).

Always sniff file data for mime type
Makes no much sense otherwise.
Credit = 2012-10-24 18:05:13 - Khaled Hosny

Add GIOGetMimeType() comment.
Credit = 2012-10-29 21:10:00 - Khaled Hosny
Fix bugs in GIOGetMimeType()

Converted sniff_length into a define because this is a compiled binary.
g_content_type_guess() was wasted compute time at the front, and the
result was thrown away, so moved it to the back as a last resort if
sniffing data for mime type failed.
Accept the possibility of "content_type==NULL" after all these check.