noorsyyed

var activityId = "059C0532-906C-E711-9409-00155D018D00";
var activityType = "appointment"; //or any other entity type

var entity = {};
entity["objectid_activitypointer@odata.bind"] = "/activitypointers(" + activityId  + ")";
entity.body = "ZGZnZA=="; //your file encoded with Base64
entity.filename = "test";
entity.subject = "test";
entity.objecttypecode = activityType;