force g_file_query_info if magic_file returns "application/octet-stream"
In rstto_file_is_valid(), ff magic_open does not recognize a file type, it will return "application/octet-stream". The existing code however seems to assume magic_open returns NULL. The patch simply checks for "application/octet-stream" and then sets content_type to NULL, triggering the function to then check the type by g_file_query_info().