Not sure if anyone else has had this issue yet, but I did. After endless searching on the internet I came up completely empty for a solution.
When I would get an email with a jpg attached, when trying to open it I would get the error message: The file cannot be opened because it is corrupted (I'm assuming pretty much any attachment would or could display this message)
Here is the fix for the problem. Keep in mind I use Exchange to sync the mail app with my Gmail account... but I'm sure this works for pop accounts too...
PLEASE MAKE SURE YOU USE SOMETHING TO BACKUP YOUR MAIL, CONTACTS AND CALENDAR FIRST! (Now, I DIDN'T backup anything and I was fine. Just don't blame me if you permanently loose something. By the time you're done here your phone should be auto magically re-downloading everything that was deleted so backup is your choice but probably not needed.)
Open up the HTC Mail app. Press Menu, then More, then Settings. Now tap DELETE ACCOUNT. This will delete the account settings and the mail box contents.
Now it should come up to the screen where you add an account. Set the account up like you originally had it. Google for the settings for your mail provider if you need help getting the correct settings.
Once the account is set up, and downloads your mail... You can now open those file attachments!
EDIT!: This fix worked at first, but now it's back to screwed again. I'm assuming it's an app problem so I'll dig in more and see if I can't find a fix...
EDIT2!: Been very busy with work this week, and still have the problem. The above procedure is still a good workaround, but a huge pain in the arse. Everytime I run into the problem, I have to do the above to get it going again. It seems that once you've done the delete account and then reset it, the first time you download an attachment it lets you open it. If you close out of the attachment then try and open it again it won't let you open it. SD Card issue maybe???
MODS - I'm not sure that the EVO Shift forum is the best place for this thread... maybe in a more general area for HTC Android users. When I searched this problem online, I saw tons of people with the problem on all kinds of HTC Android devices, but no fixes.