This command should rename files in subdirectories that don't have a suffix and don't contain a `?` character in their name by appending `.html`

append-suffix-on-files-without-suffix-in-subdirectories.sh

find . -mindepth 1 -type f -not -name "*.*" -not -name "*\?*" -exec sh -c 'mv "{}" "{}".html' \;

# In this version, I've removed the -name '*.*' option, since we want to rename files that don't have a suffix. 
# I've also replaced the -not -name '*\?' option with -not -name "*\?*", 
# which should correctly exclude files that contain a ? character in their name.