Fix form file input normalized value

lib/phoenix_html/form.ex

@@ -252,6 +252,9 @@ defmodule Phoenix.HTML.Form do
       won't be ignored on submission. This requires however
       that the textarea is rendered with no spaces after its
       content
+
+    * For "file", it returns an empty string because file path
+      can't be set programmatically.
   """
   def normalize_value("datetime-local", %struct{} = value)
       when struct in [NaiveDateTime, DateTime] do
@@ -268,6 +271,10 @@ defmodule Phoenix.HTML.Form do
     html_escape(value) == {:safe, "true"}
   end
 
+  def normalize_value("file", _value) do
+    ""
+  end
+
   def normalize_value(_type, value) do
     value
   end

test/phoenix_html/form_test.exs

@@ -115,6 +115,10 @@ defmodule Phoenix.HTML.FormTest do
       assert safe_to_string(normalize_value("textarea", nil)) == "\n"
     end
 
+    test "for file" do
+      assert normalize_value("file", "<any>") == ""
+    end
+
     test "for anything else" do
       assert normalize_value("foo", "<other>") == "<other>"
     end