I am using Endnote X1 and when I try to attach a file, I get a message ‘File not attached’. If I use the technique of trying to copy a pdf and paste into File Attachements I get the message ‘could not add link’. There is no explanation as to why in either case.
Also, when I click on the File Attachments field, I have to right click on my mouse to get the menu with the Attach File option.
It is likely that the file name is very long and/or you are so deep in the directory structure that your total filename with the path, exceed the 250ish number of characters that Windows can handle. shorten the file name, and I suspect it will work.