[KnifeLemon]

What users notice

FastTracker 2 modules (.xm) open on the audio donut and convert to every audio target (MP3, WAV, FLAC, M4A, OGG, …) through FFmpeg. Impulse Tracker (.it) and Scream Tracker 3 (.s3m) modules come along, since FFmpeg reads all three through the same demuxer. All three are read only.

FFmpeg has no tracker demuxer of its own; it needs libopenmpt (or the older libmodplug). I checked the builds Filee downloads instead of assuming:

Platform	FFmpeg	libopenmpt
Windows x64 (and ARM64 under emulation)	gyan.dev ffmpeg-9.0.2-full_build-shared.zip	yes: the README.txt inside the pinned zip lists it under external libraries
Linux x64 / ARM64	BtbN autobuild-2026-09-30-13-08 linux64-gpl / linuxarm64-gpl	yes: scripts.d/50-openmpt.sh at that tag turns it off only for winarm64
macOS	the user's FFmpeg (Homebrew)	no: homebrew-core's ffmpeg.rb has no libopenmpt or libmodplug

So an FFmpeg without either demuxer now stops a tracker module with a translated message ("This FFmpeg can't read tracker modules (XM, IT, S3M): it was built without libopenmpt. …") instead of FFmpeg's "Invalid data found". Filee asks ffmpeg -demuxers once per executable, the same way it already caches -codecs, -encoders and -filters.

Amiga .mod is not added: that extension is already the camcorder MOD video.

What changed

• src/Filee.Core/Formats/FormatRegistry.cs: xm, it, s3m as read-only audio formats.
• src/Filee.Core/Formats/FormatFields.cs: XM and IT graded specialist (audio-pro + game), S3M legacy (legacy + audio-pro, replaced by IT).
• src/Filee.Engines/Media/FfmpegConverter.cs: DemuxersOfAsync / ParseDemuxers and the tracker check in ConvertAsync and JoinAsync.
• src/Filee.Core/Settings/UserDataStore.cs, AppSettings.cs: settings schema 81 adds the three extensions to existing audio donuts; custom claims stay put. 80 is skipped because feat(image): read Windows animated cursors (.ani) #1390 already claims it, so this may need renumbering depending on merge order.
• src/Filee.App/Assets/i18n/*.json: error.tracker_needs_openmpt in all 22 languages.
• docs/ENGINES.md: the tracker paragraph under FFmpeg's read-only formats.

Tests

• tests/Filee.Core.Tests/TrackerModuleFormatTests.cs (new): detection, audio donut, schema 81 migration (learns once, leaves a user's own claim, survives a restart).
• tests/Filee.Core.Tests/FormatFieldsTests.cs: the pinned lists and grade counts.
• tests/Filee.Engines.Tests/MediaTests.cs: routes for all three modules, -demuxers parsing and the libopenmpt/libmodplug decision, and a real conversion of an XM written by the test (two channels, one pattern, a square-wave sample) to WAV, MP3 and FLAC. That test checks the length and that the WAV isn't silent. Against an FFmpeg without libopenmpt it checks Filee's message instead.
• Ran locally with gyan.dev's FFmpeg 9.0.2 full build: Filee.Core.Tests (all), the Engines tests above, Filee.App.Tests Localization*. dotnet format --verify-no-changes is clean on the touched projects.

Not covered

• macOS: untested on a Mac. The code path (refusal with Homebrew's FFmpeg) is covered by the parsing test only.
• CI doesn't install FFmpeg, so the real conversion test skips there, as the other FFmpeg tests do.

Closes #1376

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