ImageController: Update upload file helper to use interface

Related to https://github.com/symfony-cmf/MediaBundle/pull/70

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | N/A
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Merging must be coordinated with https://github.com/symfony-cmf/MediaBundle/pull/70