Do not try to add an attachment ref for external attachment if there is an internal and external attachment in the same post

boards/models/post/sync.py

@@ -334,7 +334,7 b' class SyncManager:'
                 attachment_tag.set(ATTR_ID_TYPE, ID_TYPE_URL)
                 attachment_tag.text = attachment.url
 
-        if tag_refs is not None:
+        if tag_refs is not None and attachment.is_internal():
             attachment_ref = et.SubElement(tag_refs, TAG_ATTACHMENT_REF)
             attachment_ref.set(ATTR_REF, attachment.hash)
             attachment_ref.set(ATTR_URL, attachment.file.url)