diff --git a/boards/models/attachment/__init__.py b/boards/models/attachment/__init__.py
--- a/boards/models/attachment/__init__.py
+++ b/boards/models/attachment/__init__.py
@@ -86,3 +86,6 @@ class Attachment(models.Model):
             return get_image_dimensions(preview_path)
         else:
             return 200, 150
+
+    def is_internal(self):
+        return self.url is None or len(self.url) == 0
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
@@ -50,6 +50,7 @@ ATTR_URL = 'url'
 ATTR_ID_TYPE = 'id-type'
 
 ID_TYPE_MD5 = 'md5'
+ID_TYPE_URL = 'url'
 
 STATUS_SUCCESS = 'success'
 
@@ -77,11 +78,17 @@ class SyncManager:
             if global_id.content:
                 model.append(et.fromstring(global_id.content))
                 if len(attachments) > 0:
-                    attachment_refs = et.SubElement(model, TAG_ATTACHMENT_REFS)
-                    for file in attachments:
-                        SyncManager._attachment_to_xml(
-                            None, attachment_refs, file.file.file,
-                            file.hash, file.file.url)
+                    internal_attachments = False
+                    for attachment in attachments:
+                        if attachment.is_internal():
+                            internal_attachments = True
+                            break
+
+                    if internal_attachments:
+                        attachment_refs = et.SubElement(model, TAG_ATTACHMENT_REFS)
+                        for file in attachments:
+                            SyncManager._attachment_to_xml(
+                                None, attachment_refs, file)
             else:
                 content_tag = et.SubElement(model, TAG_CONTENT)
 
@@ -114,12 +121,21 @@ class SyncManager:
 
                 if len(attachments) > 0:
                     attachments_tag = et.SubElement(content_tag, TAG_ATTACHMENTS)
-                    attachment_refs = et.SubElement(model, TAG_ATTACHMENT_REFS)
+                    
+                    internal_attachments = False
+                    for attachment in attachments:
+                        if attachment.is_internal():
+                            internal_attachments = True
+                            break
+
+                    if internal_attachments:
+                        attachment_refs = et.SubElement(model, TAG_ATTACHMENT_REFS)
+                    else:
+                        attachment_refs = None
 
                     for file in attachments:
                         SyncManager._attachment_to_xml(
-                            attachments_tag, attachment_refs, file.file.file,
-                            file.hash, file.file.url)
+                            attachments_tag, attachment_refs, file)
                 version_tag = et.SubElement(content_tag, TAG_VERSION)
                 version_tag.text = str(post.version)
 
@@ -289,14 +305,19 @@ class SyncManager:
         return signatures
 
     @staticmethod
-    def _attachment_to_xml(tag_attachments, tag_refs, file, hash, url):
+    def _attachment_to_xml(tag_attachments, tag_refs, attachment):
         if tag_attachments is not None:
-            mimetype = get_file_mimetype(file)
-            attachment = et.SubElement(tag_attachments, TAG_ATTACHMENT)
-            attachment.set(ATTR_MIMETYPE, mimetype)
-            attachment.set(ATTR_ID_TYPE, ID_TYPE_MD5)
-            attachment.text = hash
+            attachment_tag = et.SubElement(tag_attachments, TAG_ATTACHMENT)
+            if attachment.is_internal():
+                mimetype = get_file_mimetype(attachment.file.file)
+                attachment_tag.set(ATTR_MIMETYPE, mimetype)
+                attachment_tag.set(ATTR_ID_TYPE, ID_TYPE_MD5)
+                attachment_tag.text = attachment.hash
+            else:
+                attachment_tag.set(ATTR_ID_TYPE, ID_TYPE_URL)
+                attachment_tag.text = attachment.url
 
-        attachment_ref = et.SubElement(tag_refs, TAG_ATTACHMENT_REF)
-        attachment_ref.set(ATTR_REF, hash)
-        attachment_ref.set(ATTR_URL, url)
+        if tag_refs is not None:
+            attachment_ref = et.SubElement(tag_refs, TAG_ATTACHMENT_REF)
+            attachment_ref.set(ATTR_REF, attachment.hash)
+            attachment_ref.set(ATTR_URL, attachment.url)