Disable file sniffing in MIME detection on Windows.

glib on Windows doesn't do anything particularly useful with the sniffed data, and opening files on Windows can be *really* slow (especially if it's an executable/.exe; possibly because of antivirus routines).

Disabling file sniffing significantly improves the speed of the file chooser. On Windows, if `GIOguessMimeType` fails, then `GIOGetMimeType` is practically guaranteed to not help anyway.

Related issues #1434  #1640