[Core] Improves FileCollection: Now implements InputFilterProviderInterface

* Method setFileValidator to attach an validator to the FileInputs.
* Fixes Form: Inputs are attached twice for fieldset implementing InputFilterProviderInterface
* Fixes translation: Wrong strings in Validator translations.

(refs #4688)
[Applications] Adds mimetype validator to attachment file inputs.

* If configured in module.config.php key ["Applications"]["allowedMimeTypes"],
  a mimetype validator is attached to the file inputs.

(refs #4688)
Merge branch 'feature/Validate_file_uploads' into develop