Fix inputfile twoway

Fixes #750.

Adds an `isBindable()` check to the <input type="file"> path in the Two-way Binding code that probably should have always been there. Added regression test for that and all other two-way bindings. Also checked from the front that this doesn't break <input type="file"> two-way binding when a `value="{{ mustache}}"` is added, but no test for that, it's covered.