paket: How to pack asset files into nupkg so that they can be copied to output dir when referenced by client exe?

My dotnet core nupkg needs some other files to work, and I use paket add MyPkg command to add it to client project. After client project was built, I need the file structure of $(OutDir) like below:

ClientApp\bin\debug\
    | ClientApp.exe
    | MyPkg.dll
    | MyPkgAssetFolder\
         | asset_file1
         | asset_file2
         | ...

I tried to add these files into the /lib folder in the nupkg file, by editing paket.template file. But these asset files are not copied to $(OutDir) after build the client app.

How can I solve this problem?