My take on a filepath testing regular experession. Requires a relative or absolute path, and an extension. Excludes plain filenames.

filepath_regex

^[\.+]*\/+.*\.[a-zA-Z0-9]+$

# Matches
/Users/stevenschobert/Documents/project/tmp/tmp-8205.txt
../somepath.txt
./sompat/h.d
./Something with spaces/word.txt

# Does not match
../sompath
something.txt
some other /word.txt
.some other /word.txt