I posted about this previously, but apparently didn't include enough information. That thread is now closed.
This is regarding the sequence:
"Add an attachment" —> [Opens "Transaction Attachments" dialog] —> "Add" —> [Opens right click dialog] —> "Add file…"
Once upon a time, if the specified file was open in another application Quicken put up a dialog to indicate that the file was open, and to close it.
Recent versions of Quicken simply don't do anything if the file is open in another application. It would be infinitely more user friendly if Quicken were to report an error as it used to.