Why doesn't sumBy(selector) return Long?

That's a decision Kotlin team made. Since it's not possible to have return type overloads in Java the sumBy* have to have different names depending on the return type.

It's easy enough to add your own sumByLong though:

inline fun <T> Iterable<T>.sumByLong(selector: (T) -> Long): Long {
    var sum = 0L
    for (element in this) {
        sum += selector(element)
    }
    return sum
}

You can also use the sum() extension function which does have a Long variant, in conjunction with map()

val totalLength = files.map { it.length() }.sum()

Or you could wrap this into an extension function, though I don't see much benefit over the inline version:

inline fun <T> Iterable<T>.sumByLong(selector: (T) -> Long): Long {
    return map { selector(it) }.sum()
}

Optionally you can use fold function.

val totalLength = files.fold(0L) { acc, it -> acc + it.length() }

It's not much longer than sumBy code.