From jtkehler on Discord

The way special characters are replaced in filenames causes most of them to start with an underscore, which means that Anki will not delete them when checking for unused media. For example [SubsPlease] Seihantai na Kimi to Boku - 07 (1080p) [C004A6E2] becomes _SubsPlease__Seihantai_n_350092_y0xLWDQK.mp3