AnonymizeFilename: allow for a pre-defined text instead of random string

Improve docs

lib/pleroma/upload/filter/anonymize_filename.ex

@@ -1,10 +1,23 @@
 defmodule Pleroma.Upload.Filter.AnonymizeFilename do
-  @moduledoc "Replaces the original filename with a randomly generated string."
+  @moduledoc """
+  Replaces the original filename with a pre-defined text or randomly generated string.
+
+  Should be used after `Pleroma.Upload.Filter.Dedupe`.
+  """
   @behaviour Pleroma.Upload.Filter
 
   def filter(upload) do
     extension = List.last(String.split(upload.name, "."))
-    string = Base.url_encode64(:crypto.strong_rand_bytes(10), padding: false)
-    {:ok, %Pleroma.Upload{upload | name: string <> "." <> extension}}
+    name = Pleroma.Config.get([__MODULE__, :text], random(extension))
+    {:ok, %Pleroma.Upload{upload | name: name}}
+  end
+
+  defp random(extension) do
+    string =
+      10
+      |> :crypto.strong_rand_bytes()
+      |> Base.url_encode64(padding: false)
+
+    string <> "." <> extension
   end
 end