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 GNOME/gvfs!68
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.

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