Notepad cannot interpret a JPEG file, hence the reason for gobbledygook ... why that "association" has changed I do not know.
Microsoft did, some time ago, tighten up how an "attachment" in an email may be opened ... they simply closed the door on them.
Check under the Tools / Options menu, and click on the Security tab. There is a check box that reads
Do not allow attachments to be saved or opened that could potentially be a virus. Uncheck that box and try to re-open the message. You should be able to view the attachment.
If not, I'm stuck for a solution.