I'm not sure that I understand your solution or how your add-in would help. I know it's probably a user error on my side. That said, maybe if I explain what I am doing more it may give you more information to assist.

I have a folder containing PDF files of various names, but consistent naming conventions. I need to open a new email and drag and drop the pdf file. The body of the email is static except for the parts that refer to the invoice number, PO number, and Company Name. All of which are in the name of the attached PDF file. To answer your question, I don't mind an alteration to the subject as you suggested.

Again, thanks for your assistance.