Regex - File Types

^.*\.(jpg|JPG|gif|GIF|doc|DOC|pdf|PDF)$

^.+\.(?:(?:[dD][oO][cC][xX]?)|(?:[pP][dD][fF]))$

NOTE: This 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.