Keep a multi-range upload response's multipart type

A request for several byte ranges gets a 206 multipart/byteranges whose
boundary the client needs to split the body. Rewriting it to
octet-stream lost the boundary. No browser runs that type as a script,
so it passes through.

test/middleware/uploaded_script_guard_test.rb

@@ -21,6 +21,11 @@ class UploadedScriptGuardTest < ActiveSupport::TestCase
     end
   end
 
+  test "multi-range responses keep their boundary" do
+    type = "multipart/byteranges; boundary=AaB03x"
+    assert_equal type, served_type(type, "#{ActiveStorage.routes_prefix}/blobs/proxy/key/movie.mp4")
+  end
+
   test "direct upload responses keep their JSON type" do
     assert_equal "application/json", served_type("application/json", "#{ActiveStorage.routes_prefix}/direct_uploads", method: "POST")
   end