While writing an addon that adds support for additional audio formats, I noticed a strange bug? If you upload an .aac file with the mime type 'audio/x-hx-aac-adts', the XF\Http\Upload::analyzeAudio() function detects it as an .mp3 file.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.14).