Start regedit and create a backup. (file > export)
Navigate a simulair place as in the picture. Than rightclick on the 5.0 and choose new key and name it "Mail registry subkey" (without the quotes ofcourse)
Next click on that key so that it is opened. Then on the right side of the screen, right click and choose new key again. Name this one "Safe Attachments"
That will create a subkey under "Mail registry subkey"
Open that one and give it the value 0 (nul)
Now close regedit and reboot. See if that solves the problem. If not, remove the keys you just created and reboot. That will change back your system to how it was before editing.
If you feel uncomfortable, don't do it or let someone do it for you. Nevre take a risk with the registry.