diff --git a/lib/pleroma/constants.ex b/lib/pleroma/constants.ex
index bf92f65cb..343ea1acb 100644
--- a/lib/pleroma/constants.ex
+++ b/lib/pleroma/constants.ex
@@ -27,4 +27,10 @@ defmodule Pleroma.Constants do
     do:
       ~w(index.html robots.txt static static-fe finmoji emoji packs sounds images instance sw.js sw-pleroma.js favicon.png schemas doc embed.js embed.css)
   )
+
+  # basic regex, just there to weed out potential mistakes
+  # https://datatracker.ietf.org/doc/html/rfc2045#section-5.1
+  const(mime_regex,
+    do: ~r/^[^[:cntrl:] ()<>@,;:\\"\/\[\]?=]+\/[^[:cntrl:] ()<>@,;:\\"\/\[\]?=]+(; .*)?$/
+  )
 end
diff --git a/lib/pleroma/ecto_type/activity_pub/object_validators/mime.ex b/lib/pleroma/ecto_type/activity_pub/object_validators/mime.ex
new file mode 100644
index 000000000..31d51577d
--- /dev/null
+++ b/lib/pleroma/ecto_type/activity_pub/object_validators/mime.ex
@@ -0,0 +1,25 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.EctoType.ActivityPub.ObjectValidators.MIME do
+  use Ecto.Type
+
+  require Pleroma.Constants
+
+  def type, do: :string
+
+  def cast(mime) when is_binary(mime) do
+    if mime =~ Pleroma.Constants.mime_regex() do
+      {:ok, mime}
+    else
+      {:ok, "application/octet-stream"}
+    end
+  end
+
+  def cast(_), do: :error
+
+  def dump(data), do: {:ok, data}
+
+  def load(data), do: {:ok, data}
+end