Allow marking fields as file uploads

Adds a new toggle for marking a field as a file upload. If true, it sets the
`type` for the input field to `Zend\\InputFilter\\FileInput`, ensuring it can be
validated correctly within the input filter chain.

If a field returned by the API is a `FileInput`, ensures that the UI will
display it as a file upload.
Rebuilt UI distribution
Merge branch 'feature/fields-file-input'

Close #182