REST API Add attachment to Activity

Looking through the REST API documentation I can see that we can add an attachment to a custom object.

Is it supported to do so for an Activity.

WE have tried the following:

PUT https://secure-slsomxuda.crmondemand.com/OnDemand/user/Rest/028/Activities/AUDA-I33BUQ/enclosure/Attachment

Accept: application/vnd.oracle.adf.resource+json

Content-Type: application/vnd.oracle.adf.resource+json

{"Activities":[ { "Attachment":"Q3JpY2tldCBpcyBhIGJhdC1hbmQtYmFsbCBnYW1lIHBsYXllZCBiZXR3ZWVuIHR3byB0 ZWFtcyBvZiAxMSBwbGF5ZXJzIGVhY2ggb24gYSBmaWVsZCBhdCB0aGUgY2VudHJlIG9mIHdoaWNoIGlzI GEgcmVjdGFuZ3VsYXIgMjIteWFyZCBsb25nIHBpdGNoLiBFYWNoIHRlYW0gdGFrZXMgaXRzIHR1cm4gdG 8gYmF0LCBhdHRlbXB0aW5nIHRvIHNjb3JlIHJ1bnMsIHdoaWxlIHRoZSBvdGhlciB0ZWFtIGZpZWxkcy4 gRWFjaCB0dXJuIGlzIGtub3duIGFzIGFuIGlubmluZ3Mu", "AttachFileName":"Cricket", "AttachFileExt":"txt"}] }

Which returns

{ "Error":