Fix customizing Content-Type via GCS service URLs

activestorage/lib/active_storage/service/gcs_service.rb

@@ -16,7 +16,13 @@ module ActiveStorage
     def upload(key, io, checksum: nil)
       instrument :upload, key: key, checksum: checksum do
         begin
-          bucket.create_file(io, key, md5: checksum)
+          # The official GCS client library doesn't allow us to create a file with no Content-Type metadata.
+          # We need the file we create to have no Content-Type so we can control it via the response-content-type
+          # param in signed URLs. Workaround: let the GCS client create the file with an inferred
+          # Content-Type (usually "application/octet-stream") then clear it.
+          bucket.create_file(io, key, md5: checksum).update do |file|
+            file.content_type = nil
+          end
         rescue Google::Cloud::InvalidArgumentError
           raise ActiveStorage::IntegrityError
         end

activestorage/test/service/gcs_service_test.rb

@@ -35,6 +35,20 @@ if SERVICE_CONFIGURATIONS[:gcs]
       assert_match(/storage\.googleapis\.com\/.*response-content-disposition=inline.*test\.txt.*response-content-type=text%2Fplain/,
         @service.url(FIXTURE_KEY, expires_in: 2.minutes, disposition: :inline, filename: ActiveStorage::Filename.new("test.txt"), content_type: "text/plain"))
     end
+
+    test "signed URL response headers" do
+      begin
+        key  = SecureRandom.base58(24)
+        data = "Something else entirely!"
+        @service.upload(key, StringIO.new(data), checksum: Digest::MD5.base64digest(data))
+
+        url = @service.url(key, expires_in: 2.minutes, disposition: :inline, filename: ActiveStorage::Filename.new("test.txt"), content_type: "text/plain")
+        response = Net::HTTP.get_response(URI(url))
+        assert_equal "text/plain", response.header["Content-Type"]
+      ensure
+        @service.delete key
+      end
+    end
   end
 else
   puts "Skipping GCS Service tests because no GCS configuration was supplied"