Validating file types by regular expression
^.+\.(?:(?:[dD][oO][cC][xX]?)|(?:[pP][dD][fF]))$
Will accept .doc, .docx, .pdf files having a filename of at least one character:
^ = beginning of string
.+ = at least one character (any character)
\. = dot ('.')
(?:pattern) = match the pattern without storing the match)
[dD] = any character in the set ('d' or 'D')
[xX]? = any character in the set or none
('x' may be missing so 'doc' or 'docx' are both accepted)
| = either the previous or the next pattern
$ = end of matched string
Warning! Without enclosing the whole chain of extensions in (?:), an extension like .docpdf would pass.
You can test regular expressions at http://www.regextester.com/
Your regex seems a bit too complex in my opinion. Also, remember that the dot is a special character meaning "any character". The following regex should work (note the escaped dots):
^.*\.(jpg|JPG|gif|GIF|doc|DOC|pdf|PDF)$
You can use a tool like Expresso to test your regular expressions.