Fix audio/mpeg file extension mapping

[esphynox]

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Contributor checklist

• My commits are rebased on the latest main branch
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• My contribution is fully baked and is ready to be merged as is
• I have tested my contribution on these devices:

• iPhone 13 Pro, iOS 26.2

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Description

This PR fixes an incorrect file-extension mapping for the MIME type audio/mpeg, which currently resolves to .mpg and causes audio messages to be exported and shared as video files on iOS.

The mapping is updated from:

"audio/mpeg": "mpg" => "audio/mpeg": "mp3"

Context

.mpg / .mpeg refer to MPEG Program Stream container formats, which are designed to multiplex video and audio and are therefore treated as video containers by operating systems and media frameworks:

“Program stream (PS or MPEG-PS) is a container format for multiplexing digital audio, video and more.”
https://en.wikipedia.org/wiki/MPEG_program_stream

As a result, using .mpg for audio/mpeg causes exported audio files to be misclassified as video.

By contrast, the official IANA registration for audio/mpeg (RFC 3003) lists the valid file extensions as:

File extension(s): .mp1, .mp2, .mp3
https://www.iana.org/assignments/media-types/audio/mpeg

Mapping audio/mpeg to .mp3 aligns with the MIME specification and OS media classification behavior.

Result

• Audio messages with MIME type audio/mpeg are exported with the .mp3 extension
• iOS correctly recognizes them as audio files
• Sharing to Files, Notes, and other apps behaves as expected

Testing

• Sent an MP3 file to the iOS Signal app and confirmed it renders as an audio message
• Long-pressed the message and pressed Share
• Verified that file in share sheet uses the .mp3 extension and displays audio type and not .mpg extension and video type
• Confirmed no regression in in-app audio playback

Fixes #6140