To attach file to order use order/attach:

svc=order/attach&params={"itemId":<long>,
			 "id":<uint>,
			 "eventHash":<text>}

 To attach file to order send POST-request. Example is below:

Request URL:https://hst-api.example.com/example/ajax.html?svc=order/attach&sid=<sid>
Request Method:POST
Content-Type:multipart/form-data; boundary=----WebKitFormBoundarya38SwBwyXw5BFPQ1


------WebKitFormBoundarya38SwBwyXw5BFPQ1
Content-Disposition: form-data; name="params"

{"itemId":39801,"id":3,"eventHash":"jUploadForm1435580226231"}
------WebKitFormBoundarya38SwBwyXw5BFPQ1
Content-Disposition: form-data; name="eventHash"

jUploadForm1435580226231
------WebKitFormBoundarya38SwBwyXw5BFPQ1
Content-Disposition: form-data; name="file_upload"; filename="attachment.txt"
Content-Type: application/octet-stream


------WebKitFormBoundarya38SwBwyXw5BFPQ1--

{
	"error":0
}