Use of `for (prop in object)` is generally an anti-pattern, pedantic static analyzers would at least ding you on not having a `hasOwnProperty` check within the loop so that you don't get surprised by `Object.prototype` monkeys.

But more to your goal, you can trade-in 4 lines for 1 here:
```javascript
var rows = Object.keys(files).map(function(i) {return {id: files[i]};});
// yes you need both semicolons, a good linter will ding you on that inner one
```