How to create an activity attachment using WebApi in Dynamics 365

Very short post today – just wanted to share with you a piece of code which you can use to create an attachment for any activity in Dynamics 365. This is different from normal attachments, because you are not doing this by adding Annotation, rather ActivityMimeAttachment object. It’s important to remember that when you are migrating data – moving only Annotations will not transfer all Email or Appointments attachments to the new system!

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;
var req = new XMLHttpRequest();"POST", Xrm.Page.context.getClientUrl() + "/api/data/v8.2/activitymimeattachments", true);
req.setRequestHeader("OData-MaxVersion", "4.0");
req.setRequestHeader("OData-Version", "4.0");
req.setRequestHeader("Accept", "application/json");
req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
req.onreadystatechange = function() {
if (this.readyState === 4) {
req.onreadystatechange = null;
if (this.status === 204) {
var uri = this.getResponseHeader("OData-EntityId");
var regExp = /\(([^)]+)\)/;
var matches = regExp.exec(uri);
var newEntityId = matches[1];
} else {

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