Add :tmp_dir application config setting

lib/waffle/definition/storage.ex

@@ -107,6 +107,19 @@ defmodule Waffle.Definition.Storage do
   This code would authenticate request only for specific domain. Otherwise, it would send
   empty request headers.
 
+  ## Temporary Directory
+
+  When processing files, Waffle creates temporary files for operations like downloading
+  remote files or applying transformations. By default, these files are stored in the
+  system's temporary directory (as returned by `System.tmp_dir/0`).
+
+  You can customize this location by setting the `:tmp_dir` configuration option:
+
+      config :waffle,
+        tmp_dir: "/path/to/custom/tmp"
+
+  Waffle may fail to remove temporary files if the process using them crashes.
+
   """
   defmacro __using__(_) do
     quote do

lib/waffle/file.ex

@@ -134,7 +134,14 @@ defmodule Waffle.File do
       |> Base.encode32()
       |> Kernel.<>(string_extension)
 
-    Path.join(System.tmp_dir(), file_name)
+    Path.join(tmp_dir(), file_name)
+  end
+
+  defp tmp_dir do
+    case Application.get_env(:waffle, :tmp_dir) do
+      nil -> System.tmp_dir()
+      value -> value
+    end
   end
 
   defp write_binary(file) do

test/file_test.exs

@@ -0,0 +1,22 @@
+defmodule WaffleTest.File do
+  use ExUnit.Case, async: false
+
+  @custom_tmp_dir System.tmp_dir() <> "/waffle_test_custom"
+
+  describe "generate_temporary_path/1" do
+    test "uses configured tmp_dir" do
+      File.mkdir_p!(@custom_tmp_dir)
+      Application.put_env(:waffle, :tmp_dir, @custom_tmp_dir)
+
+      assert Waffle.File.generate_temporary_path() |> String.starts_with?(@custom_tmp_dir)
+      on_exit fn ->
+        Application.delete_env(:waffle, :tmp_dir)
+        File.rm_rf!(@custom_tmp_dir)
+      end
+    end
+
+    test "uses system tmp_dir" do
+      assert Waffle.File.generate_temporary_path() |> String.starts_with?(System.tmp_dir())
+    end
+  end
+end