How do you do it? The official android files trasfer app seems to transfer only files, not contacts.
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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