diff --git a/boards/management/commands/invalidate_sync_cache.py b/boards/management/commands/invalidate_sync_cache.py
new file mode 100644
--- /dev/null
+++ b/boards/management/commands/invalidate_sync_cache.py
@@ -0,0 +1,20 @@
+from django.core.management import BaseCommand
+from django.db import transaction
+
+from boards.models import GlobalId
+
+__author__ = 'neko259'
+
+
+class Command(BaseCommand):
+    help = 'Removes local global ID cache'
+
+    @transaction.atomic
+    def handle(self, *args, **options):
+        count = 0
+        for global_id in GlobalId.objects.all():
+            if global_id.is_local() and global_id.content is not None:
+                global_id.content = None
+                global_id.save()
+                count += 1
+        print('Invalidated {} caches.'.format(count))
diff --git a/boards/models/__init__.py b/boards/models/__init__.py
--- a/boards/models/__init__.py
+++ b/boards/models/__init__.py
@@ -3,8 +3,8 @@ STATUS_BUMPLIMIT = 'bumplimit'
 STATUS_ARCHIVE = 'archived'
 
 
+from boards.models.sync_key import KeyPair
 from boards.models.signature import GlobalId, Signature
-from boards.models.sync_key import KeyPair
 from boards.models.image import PostImage
 from boards.models.attachment import Attachment
 from boards.models.thread import Thread
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
@@ -43,6 +43,9 @@ ATTR_MIMETYPE = 'mimetype'
 ATTR_KEY = 'key'
 ATTR_REF = 'ref'
 ATTR_URL = 'url'
+ATTR_ID_TYPE = 'id-type'
+
+ID_TYPE_MD5 = 'md5'
 
 STATUS_SUCCESS = 'success'
 
@@ -269,6 +272,7 @@ class SyncManager:
             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_ref = et.SubElement(tag_refs, TAG_ATTACHMENT_REF)
diff --git a/boards/models/signature.py b/boards/models/signature.py
--- a/boards/models/signature.py
+++ b/boards/models/signature.py
@@ -1,5 +1,6 @@
 import xml.etree.ElementTree as et
 from django.db import models
+from boards.models import KeyPair
 
 
 TAG_MODEL = 'model'
@@ -124,6 +125,11 @@ class GlobalId(models.Model):
                             key_type=element.get(ATTR_KEY_TYPE),
                             local_id=int(element.get(ATTR_LOCAL_ID))), False
 
+    def is_local(self):
+        """Checks fo the ID is local model's"""
+        return KeyPair.objects.filter(
+                key_type=self.key_type, public_key=self.key).exists()
+
 
 class Signature(models.Model):
     class Meta: