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 837787b9f..b86ce35fd 100644
--- a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
@@ -12,14 +12,14 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidator do
   @primary_key false
   embedded_schema do
     field(:type, :string)
-    field(:mediaType, :string, default: "application/octet-stream")
+    field(:mediaType, ObjectValidators.MIME, default: "application/octet-stream")
     field(:name, :string)
     field(:blurhash, :string)
 
     embeds_many :url, UrlObjectValidator, primary_key: false do
       field(:type, :string)
       field(:href, ObjectValidators.Uri)
-      field(:mediaType, :string, default: "application/octet-stream")
+      field(:mediaType, ObjectValidators.MIME, default: "application/octet-stream")
       field(:width, :integer)
       field(:height, :integer)
     end
@@ -59,13 +59,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidator do
   end
 
   def fix_media_type(data) do
-    data = Map.put_new(data, "mediaType", data["mimeType"])
-
-    if is_bitstring(data["mediaType"]) && MIME.extensions(data["mediaType"]) != [] do
-      data
-    else
-      Map.put(data, "mediaType", "application/octet-stream")
-    end
+    Map.put_new(data, "mediaType", data["mimeType"])
   end
 
   defp handle_href(href, mediaType) do
diff --git a/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs b/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
index 0e49fda99..a72460296 100644
--- a/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
+++ b/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
@@ -27,6 +27,46 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidatorTest do
       assert attachment.mediaType == "application/octet-stream"
     end
 
+    test "works with an unknown but valid mime type" do
+      attachment = %{
+        "mediaType" => "x-custom/x-type",
+        "type" => "Document",
+        "url" => "https://example.org"
+      }
+
+      assert {:ok, attachment} =
+               AttachmentValidator.cast_and_validate(attachment)
+               |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "x-custom/x-type"
+    end
+
+    test "works with invalid mime types" do
+      attachment = %{
+        "mediaType" => "x-customx-type",
+        "type" => "Document",
+        "url" => "https://example.org"
+      }
+
+      assert {:ok, attachment} =
+               AttachmentValidator.cast_and_validate(attachment)
+               |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "application/octet-stream"
+
+      attachment = %{
+        "mediaType" => "https://example.org",
+        "type" => "Document",
+        "url" => "https://example.org"
+      }
+
+      assert {:ok, attachment} =
+               AttachmentValidator.cast_and_validate(attachment)
+               |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "application/octet-stream"
+    end
+
     test "it turns mastodon attachments into our attachments" do
       attachment = %{
         "url" =>