use value instead url in link_to body

lib/rails_admin/config/fields/types/file_upload.rb

@@ -36,7 +36,7 @@ class FileUpload < RailsAdmin::Config::Fields::Base
                 image_html = v.image_tag(thumb_url, class: 'img-thumbnail')
                 url != thumb_url ? v.link_to(image_html, url, target: '_blank') : image_html
               else
-                v.link_to(nil, url, target: '_blank')
+                v.link_to(value, url, target: '_blank')
               end
             end
           end

spec/rails_admin/config/fields/types/file_upload_spec.rb

@@ -43,4 +43,31 @@
       end
     end
   end
+
+  describe '#pretty_value' do
+    context 'when the field is not image' do
+      before do
+        RailsAdmin.config FieldTest do
+          field :string_field, :file_upload do
+            def resource_url
+              'http://example.com/dummy.txt'
+            end
+          end
+        end
+      end
+
+      let :rails_admin_field do
+        RailsAdmin.config('FieldTest').fields.detect do |f|
+          f.name == :string_field
+        end.with(
+          object: FieldTest.new(string_field: 'dummy.txt'),
+          view: ApplicationController.new.view_context,
+        )
+      end
+
+      it 'has non-empty link text' do
+        expect(rails_admin_field.pretty_value).to eq '<a target="_blank" href="http://example.com/dummy.txt">dummy.txt</a>'
+      end
+    end
+  end
 end