I’ve been busting my bottom end testing this over the last three days and still no new help from support?
:‘( :’( 
update:
shields ON mangles jpeg attachments downloaded via Thunderbird. web access to the mail account displays the jpegs fine.
Karel
With all of the avast shields, Web Shield, Mail Shield and File System Shield enabled on this win XP system with MailWasher and Outpost Firewall Pro.
I have just sent an email with an attached .jpg to my gmail account - it displayed online just fine.
I then forwarded it to another (non-gmail) email address - this was received using thunderbird 31.2.0. I then saved the attachment to my hard drive and double clicking on it, it opened without problem by the associated image viewer, IrfanView.
I’m at a loss as to what is going on with your system and avast installation - that is probably the case you have had no further input as no one has been able to replicate it.