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 c4b502cb9..f53bb02be 100644
--- a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
@@ -45,11 +45,11 @@ def fix_media_type(data) do
       data
       |> Map.put_new("mediaType", data["mimeType"])
 
-    if data["mediaType"] == "" do
+    if MIME.valid?(data["mediaType"]) do
       data
-      |> Map.put("mediaType", "application/octet-stream")
     else
       data
+      |> Map.put("mediaType", "application/octet-stream")
     end
   end