When you’ve saved a template it is added to two directories beneath the \Users folder. For me these are:
C:\Users\Huw\Documents\Visual Studio 2017\My Exported Templates
C:\Users\Huw\Documents\Visual Studio 2017\Templates\ItemTemplates
I had a few problems when adding file item templates. Sometimes the newly added templates weren’t appearing in Visual Studio. I found that an easy way to fix this is to delete the template cache. That’s the entire \ItemTemplatesCache directory here:
Then I restarted Visual Studio and all my new templates appeared in the New Item dialog.