diff --git a/boards/models/post/sync.py b/boards/models/post/sync.py
--- a/boards/models/post/sync.py
+++ b/boards/models/post/sync.py
@@ -334,7 +334,7 @@ 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)