### Resources PM @victorwu | UX @cperessini | FE @psimyn ### Problem See https://gitlab.com/gitlab-org/gitlab-ce/issues/2641, https://gitlab.com/gitlab-org/gitlab-ce/issues/24816, https://gitlab.com/gitlab-org/gitlab-ce/issues/20390 as relevant discussions. ### Description * View and add comments to a file in a merge request diff. * This applies for both text files and image files. * For example, in here (https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/4522/diffs), we would like to allow users to add comments to the _new_ images. * In here (https://gitlab.com/dimitrieh/comment-on-pictures/merge_requests/2/diffs), we would like to allow users to add comments to the _changed_ images. * The comments should be associated with an image diff or a text file diff. * The comments are resolvable, just like line diff comments. ### Design The comment thread is below the file header, but above the file content.  <img src="/uploads/e3afc3e74d74a945ebc429f4f6b44e26/merge-request__diffs-comment3.png" height="300px" /> <img src="/uploads/7e17fc2bb914aa806b376db0262f51da/merge-request__diffs-comment--add.png" height="300px" /> ### Availability and Testing Add feature spec to ensure that file header comments can be added immediately, added to a review, published and deleted.