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 bba2f5eb0..837787b9f 100644
--- a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
@@ -61,7 +61,7 @@ def url_changeset(struct, data) do
   def fix_media_type(data) do
     data = Map.put_new(data, "mediaType", data["mimeType"])
 
-    if MIME.valid?(data["mediaType"]) do
+    if is_bitstring(data["mediaType"]) && MIME.extensions(data["mediaType"]) != [] do
       data
     else
       Map.put(data, "mediaType", "application/octet-stream")
diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index 7da29b197..51c0cc860 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -203,10 +203,17 @@ def fix_attachments(%{"attachment" => attachment} = object) when is_list(attachm
 
         media_type =
           cond do
-            is_map(url) && MIME.valid?(url["mediaType"]) -> url["mediaType"]
-            MIME.valid?(data["mediaType"]) -> data["mediaType"]
-            MIME.valid?(data["mimeType"]) -> data["mimeType"]
-            true -> nil
+            is_map(url) && MIME.extensions(url["mediaType"]) != [] ->
+              url["mediaType"]
+
+            is_bitstring(data["mediaType"]) && MIME.extensions(data["mediaType"]) != [] ->
+              data["mediaType"]
+
+            is_bitstring(data["mimeType"]) && MIME.extensions(data["mimeType"]) != [] ->
+              data["mimeType"]
+
+            true ->
+              nil
           end
 
         href =