Added some code to simple forms extension

- Update config.yml.dist with missing file field and options.
- Fix the uploaded file processing (save path, generating link to file).
- File field can use mime type validator of Symfony (see: http://symfony.com/doc/current/reference/constraints/File.html#mimetypes)
- Add empty_value for choice type (see: http://symfony.com/doc/current/reference/forms/types/choice.html#empty-value)
- Add maxlength attribute for text fields.
- Submitted choice fields now saves the chosen label, not the safely converted value.