You should check out DEVONthink Pro Office. On my Synology, I keep a directory of my Blu-ray ripped files and DEVONthink indexes that directory. I’m able to make notes in the comments and tag films as well as give them a folder structure inside of DEVONthink that doesn’t exist on the Synology.
I’m not sure you can index external files with FileMaker, due to it being a true database application everything has to live inside a FMP database. That being said I only just began using FileMaker at work for a specific project, so I’m a novice.