Hello Mike,
We regret to hear about the inconvenience caused. You can add exceptions in the AVG free version, click on the link to show how to add exceptions : http://support.avg.com/SupportArticleView?urlname=How-to-make-exclusions-from-all-scans-and-shields
Feel free to let us know if you require further assistance.
Mike, Just as an aside when removing Avast Free AV did you download & run the Avast total removal tool to ensure that it was completely removed (https://www.avast.com/faq.php?article=AVKB10).
AVG Guru
I have just removed Avast Free and installed AVG Free, but the same problem is still there. Neither antivirus will let me attach files from my network drive to emails in Thunderbird. In Avast it was definitely a problem with there file screen, or whatever it was called, as switching the file screen off allowed the file to be attached. The facility to specify exceptions to the screen in Avast didn't work, the files would still not attach to the email. Has the transfer to AVG been a waste of time, as I can't see any facility on the AVG home screen to change anything at all on settings except to say whether this is a laptop or a desktop ??