@karlhursh. Please answer my question: Which email client (e.g. Outlook, Thunderbird etc) you are using on your PC?
Also which version of Avast (free or paid), and which version of Windows?
Otherwise are we to assume you are accessing Gmail via a web browser? if so which web browser (Chrome, Firefox etc)?
If Chrome, do yo have any extensions installed?
As you are suffering the same problem on your phone then are you certain it is Avast causing this?
On your PC, right click the Avast tray icon,
Click "Avast shields control" and Disable shields temporarily.
Does Gmail then work properly with attachments received with Avast turned off?
If not then your problem may be caused elsewhere.