Why ".kt" extension is sometimes visible and sometimes not?

This could happen if activity does not meet one or more of these points:

1. One activity file should contain only one class.
2. One activity file class name declaration must correspond to the good filename (filename must equals class name).
3. There are no top-level declarations (constants, variables).

If a kotlin file has exactly one outer class/object/interface as the name of the file, then Android studio just shows the name without extension.