"Attach new file" does not work if the PDF is open in another application

[AJ-learning]

As discussed here: https://forums.zotero.org/discussion/102757/zotfile-attach-new-file-function-doesnt-work

The feature when you right click on an item and click "Attach New File" does not work if the PDF file is open in a PDF viewer. The problem is that the ZotFile says the file was attached successfully when it doesn't attach at all. It should show an error message that the file is currently open in another application and that ZotFile cannot "Attach New File" until the PDF is closed from all applications.