Previously this error message was output by Rails as:

  Filename Only these extensions are supported: png, jpg, jpeg, gif,
  bmp, tiff, and ico.

This changes the message to be:

  Filename has an unsupported extension. The extension must be png, jpg,
  jpeg, gif, bmp, tiff, or ico

https://gitlab.com/gitlab-org/gitlab/merge_requests/21615#note_261398477