This behavior has been discussed a few times before with no real resolution. Have a look at:
Cannot open a JPG attachment in Outlook
thread605-766785
Outlook 2002 doesnt recognize .jpg attachments
thread605-450028
OE6 - New, Different "Can't find file" issue
thread605-757174
Operation failed with certain attachments
thread605-793789
If other attachment types seem to work correctly for you, I would say it's probably not due to a temporary environment variable. Unfortunately, I don't have a solution.