Unsetting won't work on all attributes. Currently, only
metadata attributes are supported.
https://bugzilla.gnome.org/show_bug.cgi?id=620423
* @error: a #GError, or %NULL
*
* Sets an attribute in the file with attribute name @attribute to @value.
* @error: a #GError, or %NULL
*
* Sets an attribute in the file with attribute name @attribute to @value.
+ *
+ * Some attributes can be unset by setting @attribute to
+ * %G_FILE_ATTRIBUTE_TYPE_INVALID and @value to %NULL.
*
* If @cancellable is not %NULL, then the operation can be cancelled by
* triggering the cancellable object from another thread. If the operation
*
* If @cancellable is not %NULL, then the operation can be cancelled by
* triggering the cancellable object from another thread. If the operation