Fixed #10935 -- Added UploadedFile annotation with image attribute on field clean

Added Uploaded file annotation with image attribute on field clean so it can be reused in subclasses. Also added tests and documentation for this new mini feature.
Consider appending to this paragraph (instead of the additions below): "After the field has been cleaned and validated, the `UploadedFile` object will have an additional `image` attribute containing the Pillow `Image` instance used to check if the file was a valid image. It will also update `UploadedFile.content_type` with the read image's content type, if it differs from the guessed type."