Image Pipeline: Clarify user-facing text

In the image pipeline, the error message "Incorrect" appears when the user uploads a file with an invalid name. This is not helpful and does not suggest how the user can fix their mistake.

On a successful upload, the page only shows the filename in a yellow box. There is no message that the file is ok, and the yellow colour suggests something isn't correct.

These messages should be clarified to specify what went wrong and how to fix it, or to clarify that everything is really ok and ready to upload.