I have a Mac user using Thunderbird who is having issues opening jpeg's from email attachments. She can open pdf's fine. When she clicks on the jpeg it asks her what app to use and if she picks photoshop she gets an error about invalid jpeg marker found
If she forward the email to another Mac user with Thunderbird he can open it fine.
Please be clear and specific in your reply as I am unclear on issues such as mine types, etc.