Add support for preset MIME types

It is not always possible to express custom types with a static
filename -> type mapping. Currently Plug.Static will (if the file
exists) always overwrite the header and halt the connection making it
impossible to preset or later overwrite it based on custom logic.

Thus introduce an opt-out mode to preserve whatever
`Content-Type` was set or not set before.

lib/plug/static.ex

@@ -100,9 +100,13 @@ defmodule Plug.Static do
       an enum of key-value pairs or a `{module, function, args}` to return an enum. The
       `conn` will be passed to the function, as well as the `args`.
 
-    * `:content_types` - custom MIME type mapping. As a map with filename as key
-      and content type as value. For example:
+    * `:content_types` - controls custom MIME type mapping.
+      Can be either a map with filename as key and content type as value to override
+      the default type for matching filenames. For example:
       `content_types: %{"apple-app-site-association" => "application/json"}`.
+      Or the value `false` to opt out of setting the header at all. The latter
+      can be used to set the header based on custom logic before calling this plug.
+      Defaults to an empty map `%{}`.
 
   ## Examples
 
@@ -218,6 +222,13 @@ defmodule Plug.Static do
     h in full or (prefix != [] and match?({0, _}, :binary.match(h, prefix)))
   end
 
+  defp maybe_put_content_type(conn, false, _), do: conn
+
+  defp maybe_put_content_type(conn, types, filename) do
+    content_type = Map.get(types, filename) || MIME.from_path(filename)
+    put_resp_header(conn, "content-type", content_type)
+  end
+
   defp serve_static({content_encoding, file_info, path}, conn, segments, range, options) do
     %{
       qs_cache: qs_cache,
@@ -230,10 +241,9 @@ defmodule Plug.Static do
     case put_cache_header(conn, qs_cache, et_cache, et_generation, file_info, path) do
       {:stale, conn} ->
         filename = List.last(segments)
-        content_type = Map.get(types, filename) || MIME.from_path(filename)
 
         conn
-        |> put_resp_header("content-type", content_type)
+        |> maybe_put_content_type(types, filename)
         |> put_resp_header("accept-ranges", "bytes")
         |> maybe_add_encoding(content_encoding)
         |> merge_headers(headers)

test/plug/static_test.exs

@@ -35,6 +35,28 @@ defmodule Plug.StaticTest do
     assert get_resp_header(conn, "content-type") == ["application/vnd.manifest+json"]
   end
 
+  test "overwrites Content-Type by default" do
+    conn =
+      conn(:get, "/public/fixtures/static.txt")
+      |> put_resp_header("content-type", "application/octet-stream")
+      |> call()
+
+    assert conn.status == 200
+    assert conn.resp_body == "HELLO"
+    assert get_resp_header(conn, "content-type") == ["text/plain"]
+  end
+
+  test "preserves original Content-Type if requested" do
+    conn =
+      conn(:get, "/public/fixtures/static.txt")
+      |> put_resp_header("content-type", "application/octet-stream")
+      |> call(content_types: false)
+
+    assert conn.status == 200
+    assert conn.resp_body == "HELLO"
+    assert get_resp_header(conn, "content-type") == ["application/octet-stream"]
+  end
+
   test "serves the file with a urlencoded filename" do
     conn = call(conn(:get, "/public/fixtures/static%20with%20spaces.txt"))
     assert conn.status == 200