select s,
substr(s, nullif( instr(s,'.', -1) +1, 1) )
from (
select 'no_extension_should_return_null' s from dual union
select 'another.test.1' from dual union
select 'another.test.doc' from dual union
select 'another.test.docx' from dual union
select 'another.test.stupidlng' from dual
)