diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index fda1c71df..543972ae9 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -221,9 +221,9 @@ def fix_attachments(%{"attachment" => attachment} = object) when is_list(attachm
 
         media_type =
           cond do
-            is_map(url) && is_binary(url["mediaType"]) -> url["mediaType"]
-            is_binary(data["mediaType"]) -> data["mediaType"]
-            is_binary(data["mimeType"]) -> data["mimeType"]
+            is_map(url) && MIME.valid?(url["mediaType"]) -> url["mediaType"]
+            MIME.valid?(data["mediaType"]) -> data["mediaType"]
+            MIME.valid?(data["mimeType"]) -> data["mimeType"]
             true -> nil
           end