1 extension of 'a/b/c' is ''
2 extension of 'def' is ''
3 extension of 'a.b.c' is 'c'
4 extension of 'a.b/c' is 'b/c'
6 extension of 'a.a' is 'a'
7 extension of '.gitignore' is 'gitignore'
9 extension of 'a.a' is 'a'
10 extension of 'trailing/' is ''