You can use the interval trigger. The actions would be using the shell script action to get to the directory and files you want to monitor your device should be compatible with Linux commands which can tell when stuff was modified.
cd /storage/emulated/0/download
find . -type f -name "*"
to get list all of your files names in download folder as a string and then play with it inside MacroDroid. This requires periodical checking but can be ok solution if you don't need an instantaneous trigger.