How to prevent nuget update from changing content files properties in C# project?

Question

I've added two files in my nuget package:

1. lib\net40\Binary.dll
2. content\Core.config

'Copy to Output Directory' for Core.config is setup as "Copy always". This works fine and I'm getting it in build output folder.

But whenever I update nuget package, 'Copy to Output Directory' is reverted to "Do not copy" i.e. default.

Question: How can I prevent nuget update to change properties of Content files?