Use colorbox's photo property. Example:
$('a.example').colorbox({photo:true});
The reason is that colorbox's regex to auto-detect image URLs is going to fail for that kind of URL (doesn't contain an image-type extension).
Some ideas
Change the content type to "image/jpeg"
(The caps might matter)
Add the following to the end of the url &thisisan.jpg
(Some browsers will not create an image if they don't see this at the end of the url)
Test by putting the image url directly into the browser.