Allow adding a .jpg file extension so Campfire recognizes URLs as images (redux)

Commit ebadeff1debe2e1c2a19eff15521cd753b95c559 doesn't do the trick because the greediness of the `.*` eats the extension.

c.f.:
  - http://fuckyeah.herokuapp.com/regex.jpg
  - http://fuckyeahfileextensions.herokuapp.com/regex.jpg (running 15e87ecc8c754fd22b6f5e73dc54f6fd9c8e412d)