Added capability information

scrbl/audio-decoder.scrbl

@@ -105,6 +105,24 @@ Returns the current list of supported filename extensions. This is the
 same list returned by @racket[audio-known-exts?].
 }
 
+@defproc[(audio-supported-formats)
+         (listof (cons/c string? string?))]{
+
+Returns the supported filename extensions together with the MIME type
+reported by @racketmodname[racket-mimetypes]. Unknown MIME types use
+@racket["application/octet-stream"].
+}
+
+@defproc[(audio-decoder-for-extension
+          [extension (or/c string? symbol?)])
+         (or/c #f symbol?)]{
+
+Returns the decoder selected for audio with @racket[extension], such as
+@racket['flac], @racket['opusfile], or @racket['ffmpeg]. The result is
+@racket[#f] when the extension is not recognized. Actual playback still
+sniffs the file contents before selecting its decoder.
+}
+
 @section{Built-in decoder selection}
 
 The built-in decoder table contains:
@@ -306,4 +324,4 @@ A backend integrated through this interface should provide:
 
 Once registered, files with matching extensions can be opened through
 @racket[audio-open] in the same way as the built-in FLAC, MP3,
-Opusfile, and FFmpeg backends.
+Opusfile, and FFmpeg backends.