Screenshot from the ActivityPub spec 6.12 Uploading Media _This section is non-normative._ Servers _MAY_ support uploading document types to be referenced in activites, such as images, video or other binary data, but the precise mechanism is out of scope for this version of ActivityPub. The Social Web Community Group is refining the protocol in the ActivityPub Media Upload report.
https://media.hachyderm.io/media_attachments/files/112/173/908/278/590/559/original/c35fac6c7ac271cd.png