Looks like this field is completely unused. Also, PDFPlugin sets it incorrectly, so we couldn't use this data even if we needed it.
Created attachment 201531 [details] proposed patch Let's see what EWS thinks, but I couldn't find any uses in other ports, too.
Comment on attachment 201531 [details] proposed patch Clearing flags on attachment: 201531 Committed r150064: <http://trac.webkit.org/changeset/150064>
All reviewed patches have been landed. Closing bug.