Tried it with the inspector on in Chrome and it’s an image/png binary that was sent for me. I copied the request as curl, which is sent to /api/v1/users/me/avatar/pick and it has this argument:
--data-raw $'------WebKitFormBoundarykvzARD2o9iW83dtj\r\nContent-Disposition: form-data; name="avatarfile"; filename="Screen Shot 2021-06-03 at 3.12.48 PM.png"\r\nContent-Type: image/png\r\n\r\n\r\n------WebKitFormBoundarykvzARD2o9iW83dtj--\r\n' \