Missing `type` variable which is unnecessary anyway

When trying to render form with `file` element in it, there is an error:

> Variable "type" does not exist in /home/hsz/projects/MyProject/vendor/braincrafted/bootstrap-bundle/Braincrafted/Bundle/BootstrapBundle/Resources/views/Form/bootstrap.html.twig at line 159

`file_widget` block uses `type` variable, which does not exist. However we can hardcode it to `file` because we are generating `input[type=file]`.