Add a way to sniff for a FLAC file

[padenot]

• At least two implementers are interested (and none opposed):
  • Chrome, Firefox and Safari play FLAC files
• Tests are written and can be reviewed and commented upon at:
  • I'm not sure how to test this directly, as in, check the mime-type that's been sniffed. I'm writing tests for sniffing of media files (including FLAC) here: https://bugzilla.mozilla.org/show_bug.cgi?id=1727602 (to be merged automatically to the WPT repo)
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=93887 (closed)
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1195723 (closed)
  • Safari: Haven't found it but it already works.

(See WHATWG Working Mode: Changes for more details.)

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[GPHemsley]

@padenot Note that supporting the audio/flac MIME type is not the same as supporting sniffing for content of that MIME type.

Browsers having implemented FLAC support does not inherently indicate support for sniffing it, and the linked bugs are for such general FLAC support, not explicitly for sniffing.

While I do see that sniffing was added alongside the Firefox implementation (1ce419e3955f, e42414eeeabe), I am not seeing that in the Chrome/Chromium implementation. Are you able to point more directly to where Chrome/Chromium is doing the sniffing?

Separately, your own recent Firefox bugs/commits on this topic may also be of relevance:
https://bugzilla.mozilla.org/show_bug.cgi?id=1704115
https://bugzilla.mozilla.org/show_bug.cgi?id=1727361

[GPHemsley]

I think I found where Chromium is sniffing for FLAC, which seems to predate its adding support for FLAC and also uses the MIME type audio/x-flac:
https://chromium.googlesource.com/chromium/src.git/+/785d871db8a39205b3c138d018b835a6b6a2ba41%5E%21/

It doesn't appear to have changed since 2013:
https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/net/base/mime_sniffer.cc
https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/net/base/mime_sniffer_unittest.cc

I don't know the ramifications of this.

@annevk Does seem like enough information to merge this?

[annevk]

If you navigate to a FLAC file directly (or in an iframe) and it doesn't have Content-Type set or to a bogus value, does doc.contentType reveal it? Or is it only sniffed by media elements and not navigation?

[GPHemsley]

In a browsing context, Chrome identifies this as text/plain and Firefox identifies this as audio/flac.

[annevk]

And Chrome also tries to render it as text and not offer playback?

[GPHemsley]

Yeah, Chrome just displays the fLaC text.