Revert 855208a293.

This causes conflicts with the way the instance variable `@file` in the
test file works with the scope of `mock_app`. Without assigning the
`@file` instance variable to a temporary variable—in this case `path`—we
won't be able to access the instance var inside the mock_app > get context.

test/helpers_test.rb

@@ -778,9 +778,10 @@ class HelpersTest < Minitest::Test
     end
 
     def send_file_app(opts={})
+      path = @file
       mock_app {
         get '/file.txt' do
-          send_file @file, opts
+          send_file path, opts
         end
       }
     end
@@ -882,10 +883,11 @@ class HelpersTest < Minitest::Test
     end
 
     it "does not override Content-Type if already set and no explicit type is given" do
+      path = @file
       mock_app do
         get('/') do
           content_type :png
-          send_file @file
+          send_file path
         end
       end
       get '/'
@@ -893,10 +895,11 @@ class HelpersTest < Minitest::Test
     end
 
     it "does override Content-Type even if already set, if explicit type is given" do
+      path = @file
       mock_app do
         get('/') do
           content_type :png
-          send_file @file, :type => :gif
+          send_file path, :type => :gif
         end
       end
       get '/'
@@ -904,9 +907,10 @@ class HelpersTest < Minitest::Test
     end
 
     it 'can have :status option as a string' do
+      path = @file
       mock_app do
         post '/' do
-          send_file @file, :status => '422'
+          send_file path, :status => '422'
         end
       end
       post '/'