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

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).

Solution :

One activity file should contain only one class.

One activity file class name declaration must correspond to the good filename (filename must equals class name).