.opus file extension changes to .ogg in Opus file attachments

[Aivaras]

Affected version

v2.2.17, including the current version of xenforo.com

When I attach a file with the .opus extension to a post, the extension gets changed to .ogg during the process. If I then download the .ogg file, it still plays back as Opus. So it seems that XF automatically encapsulates Opus files in the OGG container. Why does this happen, and can it be avoided? We use Opus files as digital products, and changing their extensions is very misleading.

A possible cause of the problem may be 'opus' => 'audio/ogg' instead of 'opus' => 'audio/opus' in the snippet below:

Fixed Thread 'Fix detection of Opus audio files with .opus instead of .ogg extension'

Sep 29, 2021

Opus audio files can be uploaded (and rendered as an audio player) when they have a ".ogg" extension. XenForo doesn't recognize them when they have the recommended ".opus" extension (https://en.wikipedia.org/wiki/Opus_(audio_format)). This is easy to fix.

diff --git a/src/XF/App.php b/src/XF/App.php
index 51395a939..7cf644589 100644
--- a/src/XF/App.php
+++ b/src/XF/App.php
@@ -349,6 +349,7 @@ class App implements \ArrayAccess
             return [
                 'mp3' => 'audio/mpeg',
                 'ogg' => 'audio/ogg',
+                'opus' => 'audio/ogg'...

• Steffen
• Replies: 11
• Forum: Resolved bug reports

 

[Kirby]

So it seems that XF automatically encapsulates Opus files in the OGG container.

I dont think so, XenForo doesn't modify the file - it just changes the extension.

Why does this happen, and can it be avoided?

Opus (audio format) - Wikipedia

en.wikipedia.org

Opus packets are not self-delimiting, but are designed to be used inside a container of some sort which supplies the decoder with each packet's length. Opus was originally specified for encapsulation in Ogg containers, specified as audio/ogg; codecs=opus, and for Ogg Opus files the .opus filename extension is recommended.

So most likely your .opus files are Ogg containers that contain opus audio.
When XenForo detects an Ogg container, it automatically changes the fie extension to .ogg.
This isn't wrong technically as it is an Ogg container after all, but I understand why this may be confusing.

To avoid this you can add a little patch to XF/Http/Upload.php

Add

// check if it is opus which has recommended file extension .opus and set this instead of .ogg
$first64 = substr($preamble, 0, 128);
if (strpos($first64, '4F70757348656164') !== false)
{
    $audioType = 'opus';
}

after

$audioType = 'ogg';

and .opus files that are Ogg containers with opus audio should stay .opus (but .ogg files with opus audio will be changed to .opus).

A possible cause of the problem may be 'opus' => 'audio/ogg' instead of 'opus' => 'audio/opus' in the snippet below:

audio/ogg is the correct MIME type for an ogg container with opus audio.

 

[Aivaras]

Hi Kirby,

Thank you very much for your insights, suggestions, and explanations—they’re highly appreciated!

So most likely your .opus files are Ogg containers that contain opus audio.

Yes, FFprobe (FFmpeg) reports that the files are contained in OGG, as expected. That’s not the issue. The issue lies with the file extension.

When XenForo detects an Ogg container, it automatically changes the fie extension to .ogg.
This isn't wrong technically as it is an Ogg container after all, but I understand why this may be confusing.

I’m not sure I can agree here (the “isn’t wrong” bit).

1) See the Official Opus file format specification, RFC 7845 > Section 9: Content Type:

RFC 7845: Ogg Encapsulation for the Opus Audio Codec

This document defines the Ogg encapsulation for the Opus interactive speech and audio codec. This allows data encoded in the Opus format to be stored in an Ogg logical bitstream.

datatracker.ietf.org

which clearly states:

The RECOMMENDED filename extension for Ogg Opus files is '.opus'.

2) In the audio world—including professional audio production and authoring software—the .ogg extension is reserved for Ogg Vorbis, a completely different codec. While still widely used, Vorbis is deprecated, and Opus is its successor.

audio/ogg is the correct MIME type for an ogg container with opus audio.

Yes, this is in line with RFC 7845.

 

[Kirby]

In the audio world—including professional audio production and authoring software—the .ogg extension is reserved for Ogg Vorbis

I don't see anything in the linked RFC that backs your claim, it is only mentioned that .opus is recommended for ogg opus (and .ogg for ogg vorbis according to RFC5334).
So unless there is some statement in this (or another RFC) that kinda enforces .opus via MUST I'd say XF use is kinda acceptable, though not ideal.