Python: Check if uploaded file is jpg

How can I check if a file uploaded by a user is a real jpg file in Python (Google App Engine)?

This is how far I got by now:

Script receives image via HTML Form Post and is processed by the following code

...
incomming_image = self.request.get("img")
image = db.Blob(incomming_image)
...

I found mimetypes.guess_type, but it does not work for me.