gfile: Fallback to fast-content-type if content-type is not set
The G_FILE_ATTRIBUTE_STANDARD_CONTENT_TYPE attribute doesn't have to be always set. See gvfs!68 (merged) for more details. In that case, the g_file_query_default_handler function fails with the "No application is registered as handling this file" error. Let's fallback to the "standard::fast-content-type" attribute instead to fix issues when opening such files.
/cc @hadess