Use absolute filenames and include some comments:

# my first filename
file /mnt/share/file-1.wav
# my second filename including whitespace
file '/mnt/share/file 2.wav'
# my third filename including whitespace plus single quote
file '/mnt/share/file 3'\''.wav'

Allow for input format auto-probing, use safe filenames and set the duration of the first file:

ffconcat version 1.0
file file-1.wav
duration 20.0
file subdir/file-2.wav