How do you do it? The official android files trasfer app seems to transfer only files, not contacts.
In fact, I've found the way.
First you have to backup your contacts within your phone. They'll be saved as .vcf file. <<< this is the main point.
Then you can use the officia android file transfer application. You'll see the file once you can see phone's folders.
Side note: the application can't see your phone, first activate the USB debug from the developer options.
Hope it helps