DEV: use cdn url to download the external uploads to local.

lib/file_store/base_store.rb

@@ -77,7 +77,8 @@ def download(upload)
 
         if !file
           max_file_size_kb = [SiteSetting.max_image_size_kb, SiteSetting.max_attachment_size_kb].max.kilobytes
-          url = SiteSetting.scheme + ":" + upload.url
+          url = Discourse.store.cdn_url(upload.url)
+          url = SiteSetting.scheme + ":" + url if url =~ /^\/\//
           file = FileHelper.download(
             url,
             max_file_size: max_file_size_kb,

spec/components/file_store/base_store_spec.rb

@@ -59,19 +59,33 @@
     end
 
     let(:upload_s3) { Fabricate(:upload_s3) }
+    let(:store) { FileStore::BaseStore.new }
 
     it "should return consistent encodings for fresh and cached downloads" do
       # Net::HTTP always returns binary ASCII-8BIT encoding. File.read auto-detects the encoding
       # Make sure we File.read after downloading a file for consistency
 
-      store = FileStore::BaseStore.new
-
       first_encoding = store.download(upload_s3).read.encoding
 
       second_encoding = store.download(upload_s3).read.encoding
 
       expect(first_encoding).to eq(Encoding::UTF_8)
       expect(second_encoding).to eq(Encoding::UTF_8)
     end
+
+    it "should return the file" do
+      file = store.download(upload_s3)
+
+      expect(file.class).to eq(File)
+    end
+
+    it "should return the file when s3 cdn enabled" do
+      SiteSetting.s3_cdn_url = "https://cdn.s3.amazonaws.com"
+      stub_request(:get, Discourse.store.cdn_url(upload_s3.url)).to_return(status: 200, body: "Hello world")
+
+      file = store.download(upload_s3)
+
+      expect(file.class).to eq(File)
+    end
   end
 end