Edit Files API Annotation

This corrects the possible values for the encoding parameter and default (as per https://docs.gitlab.com/ee/api/repository_files.html#create-new-file-in-repository) to be included in autogenerated grape-swagger OpenAPI documentation. Otherwise, the documentation will only allow base64 encoding.

The OpenAPI V2 documentation will be used for dogfooding API fuzzing as per #372422 (closed) and the OpenAPI autogeneration &8926.