Add tests

diff --git a/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
index d65a6f2..99f7e28 100644
--- a/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs

@@ -42,6 +42,7 @@
 using static UnitTest.Issues.TestProtos.UnittestCustomOptionsProto3Extensions;
 using static UnitTest.Issues.TestProtos.DummyMessageContainingEnum.Types;
 using Google.Protobuf.TestProtos;
+using Google.Protobuf.TestProtos.Extensions;
 
 #pragma warning disable CS0618
 
@@ -193,6 +194,12 @@
             Assert.NotNull(TestAllTypes.Descriptor.Oneofs[0].CustomOptions);
         }
 
+        [Test]
+        public void MultipleImportOfSameFileWithExtension()
+        {
+            var descriptor = ExtensionsCReflection.Descriptor;
+        }
+
         private void AssertOption<T, D>(T expected, OptionFetcher<T> fetcher, Extension<D, T> extension, Func<Extension<D, T>, T> descriptorOptionFetcher) where D : IExtendableMessage<D>
         {
             T customOptionsValue;

diff --git a/csharp/src/Google.Protobuf.Test/testprotos.pb b/csharp/src/Google.Protobuf.Test/testprotos.pb
index 818b227..b1c6650 100644
--- a/csharp/src/Google.Protobuf.Test/testprotos.pb
+++ b/csharp/src/Google.Protobuf.Test/testprotos.pb
Binary files differ