+---------------+
|abrahadabra.pdf|
|---------------|
|some.txt       |
|---------------|
|Doc.docx       |
+---------------+

+------+
|.pdf  |
|------|
|.txt  |
|------|
|.docx |
+------+

SELECT  ( SELECT (substr( d.DOC_FNAME, length(d.DOC_FNAME) - instr('.', d.DOC_FNAME)+1)) FROM dual) AS doctype
FROM Documents d
GROUP BY d.DOC_FNAME;

select substr(DOC_FNAME,-instr(reverse(DOC_FNAME),'.'))
  from table

select regexp_substr(DOC_FNAME,'.[^.]*$')
  from table