image saving with basic error handling

test/file_server/file_test.exs

@@ -0,0 +1,13 @@
+defmodule PlantIdDiscordBotTest.FileServer.File do
+  use ExUnit.Case
+  doctest PlantIdDiscordBot.FileServer.File
+
+  @image_path Application.compile_env(:plantid_discord_bot, :image_path)
+
+  test "generate_unique_filename" do
+    file_path = PlantIdDiscordBot.FileServer.File.generate_unique_filename("jpg")
+    file_name = Path.basename(file_path)
+    assert Regex.match?(~r/^[0-9A-F]{16}\.jpg$/, file_name)
+    assert file_path == Path.join(@image_path, file_name)
+  end
+end