Allow *.spx glob for audio/x-speex+ogg

Currently, *.spx is associated with "raw" speex, i.e. without an ogg container, while it is also often used for audio/x-speex+ogg. The latter currently only allows the *.oga and *.ogg globs.

See also https://bugs.freedesktop.org/show_bug.cgi?id=5299 https://bugs.freedesktop.org/show_bug.cgi?id=53376

https://speex.org/docs/manual/speex-manual/node8.html says:

For now, you should use the MIME type audio/x-speex for Speex-in-Ogg. We will apply for type audio/speex in the near future.

The libspeex speexenc/speexdec use ogg containers, and use the *.spx extension, see https://linux.die.net/man/1/speexdec

"raw" speex and speex+ogg are covered by their content matches, i.e. 'OggS'@0 + 'Speex '@28 vs. 'Speex '@0.

Apparently the only application capable of creating "raw" speex is the xiph sampleenc, see https://github.com/xiph/speex/blob/master/doc/sampleenc.c