Improve Postcript files support

Hi,

Improvements are as follows :
- Adding an options array value 'ps_file_extensions' to list all the Postcript extensions and prevent hard coded value in code,
- Adding 2 methods to check if file is Postcript (based on the newly created options 'ps_file_extensions') and to add PNG extension to postcript file
- Fix bug while deleting PS images & thumbnails (only PS file was deleted)

Benefits:
- Only one place to edit PS extensions
- Full PS files deletion
- Code more readable

Tested with all listed PS files, with or without Imagick