Commit a8ae56e06a ("Part: Rename AttachExtension::Support property to
AttachmentSupport...") provided code to convert property name when loading
files saved with previous versions. Unfortunately 84ae6b9b7b
("Toponaming: bring in missing code fragments in Part") was based on
at the time obsolete code which did not contain before mentioned tweak,
so AttachmentSupport ended unassigned.
Fixes: 84ae6b9b7b ("Toponaming: bring in missing code fragments in Part")