Asset key in public API

[bulich]

What is the best way to add item image thumbnail and get an acces to it in items public API without any extra requests? For example, I have a simple posts collection (title, subtitle and thumbnail image). If i use file field type then I get only ID of this image file:

{
 id: 1,
 title: 'Some title',
 subtitle: 'Some subtitle',
 thumbnail: 1
}

But I need a private hash key to get an asset from my frontend side (GET /:project/assets/:key).