Safari on iOS and OS X relies on the HTTP Content-Range header for media files. Currently, this mainly affects audio files served from attachments on iOS, where MP3 files cannot search, and MP4 just doesn't work. If using an audio tag —
PHPBB3-14771 — only MP3 works in Safari.
The cause of this is that the Content-Range support in phpBB is tailored for resuming downloads; any range ending before end-of-file is silently dropped and the full file is served instead.