If a FileSet's title property isn't set, it should delegate title to its original_file's filename

[escowles]

You can set a title property on a FileSet, but it's common to have this simply be the original_file's filename. This can get out of sync (e.g., if you replace the content with a file with a different file, the title isn't updated).

We should not set the title explicitly, and if there is no title property set, then we should return the original_file's filename value.