Atari 2600

This core has been superseded by Atari 7800 core. 2600 repository disabled from further updates and now is read-only. All further bug fixes and improvements for Atari 2600 should be made in Atari 7800 core which is fully compatible with 2600. Also 7800 core has improvements in 2600 part. All supported mappers are automatically detected and don't require special file extensions. So simply rename them back to standard a26 or bin extension.
Atari 2600 core still can be downloaded manually from its repository in its current form.