Update fields.html.twig for better file support by adding file_widget

I wasn't happy with the field output for bootstrap 3 for file upload, so I've added a widget for file that implements the default file input from bs3.  I pretty much just copied pieces from other widgets here, so there may be parts that shouldn't be there or others that are missing.  However, it works for my needs so I'm submitting this pull request.
Feel free to comment and I'll fix anything that I didn't understand :smile: