All valid file types should have file type strings (and, currently, they

All valid file types should have file type strings (and, currently, they
all do); get rid of the test for a null return from
wtap_file_type_string().)

If wtap_get_file_extensions_list() returns NULL, include the file type
in the list of filters, and use "*.*" as the filter.  That way the list
of filters will include all file types, even if you can't really ask
only for files of that type (actually, you can't really ask only for
files of *any* type unless you're running under a desktop environment
where file types are specified by, for example, looking for magic
numbers, as there's no guarantee that, for example, a pcap file will
have an extension at all, given that it might come from a command-line
tool that doesn't default to any extension).

svn path=/trunk/; revision=40650