Add a direct_upload: true option to file_field_tag and Form#file_field

Then users don't have to bother with manually referring to internal routes.

README.md

@@ -116,7 +116,7 @@ Active Storage, with its included JavaScript library, supports uploading directl
 2. Annotate file inputs with the direct upload URL.
 
     ```ruby
-    <%= form.file_field :attachments, multiple: true, data: { direct_upload_url: rails_direct_uploads_url } %>
+    <%= form.file_field :attachments, multiple: true, direct_upload: true %>
     ```
 3. That's it! Uploads begin upon form submission.
 

app/helpers/active_storage/file_field_with_direct_upload_helper.rb

@@ -0,0 +1,18 @@
+module ActiveStorage
+  # Temporary hack to overwrite the default file_field_tag and Form#file_field to accept a direct_upload: true option
+  # that then gets replaced with a data-direct-upload-url attribute with the route prefilled.
+  module FileFieldWithDirectUploadHelper
+    def file_field_tag(name, options = {})
+      text_field_tag(name, nil, convert_direct_upload_option_to_url(options.merge(type: :file)))
+    end
+
+    def file_field(object_name, method, options = {})
+      ActionView::Helpers::Tags::FileField.new(object_name, method, self, convert_direct_upload_option_to_url(options)).render
+    end
+
+    private
+      def convert_direct_upload_option_to_url(options)
+        options.merge('data-direct-upload-url': rails_direct_uploads_url) if options.delete(:direct_upload)
+      end
+  end
+end