All music file types, although I use only Flac and MP3 on my DX50. I have re-ripped all my old Alac and mp4 Apple rips. I agree that it will read longer filenames on the fly. What I am asserting and from my experience with a 64gb Sandisk card, is that if you want a one time scan that yields all your albums and not requiring multiple scans and ad hoc scans, reducing the filename length will solve this. With the protocol of 30 characters, my DX50 scans properly and completely every time. Introduce just one delinquent filename, and it won't scan thoroughly. Because about 1000 of my tracks were classical and extremely long, it was worth my while to do this because I simply could not get my albums to appear even after many scans, most of which didn't yield any new albums. If they ever fix this at iBasso, it won't take long to rename them with MP3Tag so its no big deal IMHO.
I choose my playlists from Album view mostly, but even in Directory view, 30 characters is adequate to identify tracks.
I followed a similar procedure with my classical FLAC collection using mp3tag. I stripped all tags and reduced file names to 40 characters. No problems after that. I'm using Sandisk 64GB cards (ExFat format) and use Album view to access files.