When you view source of the message (right-click -> view all headers and message), in the headers for the attachment, look at the attachment's Content-Type, Content-Disposition and Content-Transfer-Encoding headers. What do you see?

It should be something like:

Content-Type: image/jpeg; name=test.jpg Content-Disposition: attachment; filename=test.jpg Content-Transfer-Encoding: base64

What do you see?

I'm using Opera Mail

The standalone Opera Mail 1.0 or Opera Mail that's built into Opera 12.x?