Write MediaDescriptor back

Otherwise setting the filter name has no effect.

Change-Id: I333c8d2be66537a2c83b2aa3e40cdef61a5ac970
related: fdo#67104 correct the tooltip

This option points to the per-sheet setting, not to the global one.

Change-Id: I79dbd727e0ff12eb906f243f82f4906762918d26