What is the best way for working with files in kotlin? Do I still need to use FileInputStream and other classes from Java?

Yes.

Yes, but you can use standard extension functions that make this easier. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.io/index.html#extensions-for-external-classes

I also wrap functions that work on files in a suspend fun

for simple things like readText, writeText, useLines, mkdirs, etc (which is really all i needed to use so far) you do not need to fall back to java i hope those will soon be available in a multiplatform library

Thanx guys

I also wrap functions that work on files in a suspend fun

Theoretically these should run in an IO dispatcher...