> . It seems not possible to force the Travis switch the platform from 18 to 25 because running on the nightly will make Travis busted easily. 

see https://bugzilla.mozilla.org/show_bug.cgi?id=898516

I get the problem with the current travis though. The best way I think of is stubbing `toBlob` using sinon, so that you can really control what's going on. With sinon's stub API (see http://sinonjs.org/docs/#stubs) you can eg ask to call the callback with a parameter you build yourself.

This is probably the way to go instead of using several media. This way you can really test all branches in your code :)