[Endnote XML] Export file attachments in EndNote format

Re https://forums.zotero.org/discussion/26493/endnote-xml-exportimport/#Item_20

This should be mostly beneficial, since I think most of these exports would go to EndNote, rather than some other program. If it's a new library, user would just have to copy the PDF folder to their library directory. If it's an existing library, then you need to merge the contents of the PDF folder. (you could probably perform Zotero export directly into EndNote's library directory)