Implement it as a user-defined Blob subtype.

Change-Id: Ia369b6858e7d9191f34032445c1003931273e926
Reviewed-on: https://gerrit.libreoffice.org/47098Reviewed-by: Tamás Bunth <btomi96@gmail.com>
Tested-by: Tamás Bunth <btomi96@gmail.com>