Restore Missing fileInputId Property and Add acceptedFileTypes Property

[chadrschroeder]

Fixes #1088 by restoring the missing fileInputId property.

Also adds an acceptedFileTypes property, similar to what was proposed in https://github.com/basecamp/trix/pull/789. That PR was opened over 3 years ago back when this code was still using CoffeeScript. I've implemented a similar change here in JavaScript, but I set this up as a simple string attribute value instead of an array. Let me know if you'd rather not have this option, but this is why I was trying to override the pickFiles function in the first place.

[chadrschroeder]

Thanks for the quick review before. Let me know if there's anything else you want me to update here. It looks like the most recent tests failed but in a way that is unrelated to my changes.

[ghiculescu]

Is this okay to merge @afcapel ?

[afcapel]

@ghiculescu I afraid I don’t maintain Trix anymore, so that ‘s up to the new maintainers.