Fix .jpeg precompile issue

Fixes rails/sprockets-rails#304

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprockets/blob/master/UPGRADING.md
 
+- Fix for precompile issues when multiple extensions map to the same MIME type (eg. `.jpeg` / `.jpg`). [#781](https://github.com/rails/sprockets/pull/781)
+
 ## 4.2.0
 
 - Rack 3 compatibility. [#758](https://github.com/rails/sprockets/pull/758)

lib/sprockets/loader.rb

@@ -165,7 +165,9 @@ def load_from_unloaded(unloaded)
         end
 
         if type = unloaded.params[:type]
-          logical_path += config[:mime_types][type][:extensions].first
+          extensions = config[:mime_types][type][:extensions]
+          extension = extensions.include?(extname) ? extname : extensions.first
+          logical_path += extension
         end
 
         if type != file_type && !config[:transformers][file_type][type]

test/test_loader.rb

@@ -39,6 +39,14 @@ def setup
     assert_raises Sprockets::ConversionError do
       @env.load("file://#{fixture_path_for_uri('default/gallery.js')}?type=text/css")
     end
+
+    assert asset = @env.load("file://#{fixture_path_for_uri('default/blue_jpg.jpg')}?type=image/jpeg")
+    assert_equal fixture_path('default/blue_jpg.jpg'), asset.filename
+    assert_equal 'blue_jpg.jpg', asset.logical_path
+
+    assert asset = @env.load("file://#{fixture_path_for_uri('default/blue_jpeg.jpeg')}?type=image/jpeg")
+    assert_equal fixture_path('default/blue_jpeg.jpeg'), asset.filename
+    assert_equal 'blue_jpeg.jpeg', asset.logical_path
   end
 
   test 'load outside asset' do