It seems as if the uploadMedia ActivityPub extension does not provide a way to resume an upload that didn't previously compete.
https://www.w3.org/wiki/SocialCG/ActivityPub/MediaUpload
If, for example, you are working with large files (such as video files) this would matter.
Because if you uploaded 1GB, and the upload stopped, you would want to resume at where it stopped, and not have to upload from the beginning again.
This would be important for ActivityPub C2S adoption.