Added attachment refs to the protocol sample

docs/dip-1.markdown

@@ -144,13 +144,7 b' Sample response:'
                         signature.
                     -->
                     <attachments>
-                        <attachment mimetype="image/png" name="12345.png">
-                            TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0
-                            aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1
-                            c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0
-                            aGUgY29udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdl
-                            LCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=
-                        </attachment>
+                        <attachment mimetype="image/png">TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5I</attachment>
                     </attachments>
                 </content>
                 <!--
@@ -161,6 +155,10 b' Sample response:'
                     <signature key="id1" type="ecdsa" value="dhefhtreh" />
                     <signature key="id45" type="ecdsa" value="dsgfgdhefhtreh" />
                 </signatures>
+                <attachment-refs>
+                    <attachment-ref ref="TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0"
+                        url="/media/images/12345.png" />
+                </attachment-refs>
             </model>
             <model name="post">
                 <content>