Bug 990481 - [ZTE][OPENC]MMS app allows to attach more contents even whe...

...n the max MMS size is reached
Please, use ```this.sinon.spy``` here, in this case you won't need ```teardown``` function below at all, cause ```restore``` is done automatically! You can read more about ```sandboxing``` in [1].

[1] http://sinonjs.org/docs/#sandbox