Don't expand image URLs that don't really point to images.

Sometimes a URL will end with an image extension but points to something else instead, such as Wikimedia image summary pages. Limechat will try to thumbnail these resulting in a broken image inserted into the log. I added a Content-Type check to mitigate this.

![Screen Shot 2013-03-01 at 10 47 07 AM](https://f.cloud.github.com/assets/936355/210953/c2d1b2cc-82a3-11e2-8682-03e4bc5106f2.png)