diff --git a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
index 8b641d88d..cf6e407e0 100644
--- a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
@@ -59,7 +59,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidator do
   end
 
   def fix_media_type(data) do
-    Map.put_new(data, "mediaType", data["mimeType"])
+    Map.put_new(data, "mediaType", data["mimeType"] || "application/octet-stream")
   end
 
   defp handle_href(href, mediaType, data) do