Old school Photo Edit.

Sounds more like a Windows forum question then a Firefox since it sounds like a Windows operation here. As to hotmail not receiving that could be your hotmail isn't the recognized email for youtube account for verification.

When/where do you notice this change to the file extension?

Note that these days the file extension you see in a URL (link) doesn't mean that much as websites can change the image format on the fly to a format they think that the browser supports due to user agent sniffing and the HTTP accept header.

You can inspect the MIME database key with the registry editor (regedit.exe) and do a search for that MIME type (file extension) via Ctrl+F. Be cautious with editing the registry as there is NO UNDO possible: all changes are applied immediately. You can export key(s) in the registry editor before making changes. You can check specific file extension keys (e.g. .jpg or .jpeg) in the registry with the registry editor.

• HKEY_CLASSES_ROOT\.xxx

You can check possibly linked MIME types in the MIME Database registry key.

• HKEY_CLASSES_ROOT\MIME\Database\Content Type\