|After you open the file, click File Save As and check the window that opens. |
If I understand the process correctly, Outlook detaches and saves the file to a temp directory before it opens it. For example on my machine, the current location is C:\...Temporary Internet Files\OLK5
If the previous file e.g. A.xls is still in that folder, Outlook wil save the next A.xls as A (2).xls so that it doesn't overwrite the existing file.
As far as I know, the only ways to stop getting the (2) etc. is to either manually rename/delete the file in the temp directory or quit Outlook so it clears the directory itself.