From what I've found, Radarr tends to want a fairly strict naming convention for the folder name, e.g. "It's a Wonderful Life (1946)". If it's not movie title (year) then it just tends to ignore it on a scan.
I've got a few foreign language movies that I've titled "English Title [Original Title] (year)" and it will not see them. I have to add them manually, then when I'm on that movie in Radarr, press the refresh button and it'll find the file.
They can't be nested in sub-folders either - I originally had stuff like Movies\James Bond\Dr. No (1962), but had to change that to Movies\Dr. No (1962)
Somebody may correct me here, but I *think* it uses themoviedb as its reference for the correct title and year of a movie. If your folder is named the same as on that site, you should be good. If the title has illegal characters in it - colon, slashes, quotes - then Radarr seems to remove them. It seems OK if you replace them with dashes, which I often do. Also, the year is sometimes different on themoviedb than it is on imdb. Not sure whether that makes a difference.
That's just folder formatting - it doesn't seem to be too concerned with filenames, but I'm not sure about recognised file formats.
When I originally installed Radarr, I scanned my movie folder and assumed that it picked everything up. Years later I still find movies in my collection that it's not aware of. Wish there was an easy way for Radarr to tell you everything it has ignored, so you could at least do something about it.