Downloading is possible for both photos and videos from posts, as well as for stories, reels or profile picture.
Do you mean add the custom format to rename file to include resource index number?
Yes, right now the only way to have each download with a unique filename is to include the ORIGINAL_NAME or ORIGINAL_NAME_FIRST custom formats, but that can give you a very long filename some times, and those are also not related to the order of pics/videos in a post.
Would it be possible to add a Custom Format to the list to add the index of the specific media to be downloaded (in post URL it's the value in "?img_index=")? It would be very useful to download multiple media from the same post and having the names ending with _1, _2 and so on.