MIME: added audio/flac type.

Chrome and other browsers implementing Opaque Request Blocking block
`<audio>` elements from playing if it cannot guess it is an audio file
and the Content-Type header does not say it is that [1]. This is
currently the case for FLAC files served by Nginx instances configured
to use the included `mime.types` file. Chrome is unable to
guess a file's type if the server responds with 206 Partial Content,
which is typically the case for FLAC files.

The `audio/flac` MIME type is officially registered by IANA [2].
FLAC files are supported by browsers used by 97.87% of the world's
population [3].

[1]: https://groups.google.com/a/chromium.org/g/blink-dev/c/ScjhKz3Z6U4/m/5i_0V7ogAwAJ
[2]: https://www.iana.org/assignments/media-types/audio/flac
[3]: https://caniuse.com/flac

conf/mime.types

@@ -78,6 +78,7 @@ types {
     application/octet-stream                         iso img;
     application/octet-stream                         msi msp msm;
 
+    audio/flac                                       flac;
     audio/midi                                       mid midi kar;
     audio/mpeg                                       mp3;
     audio/ogg                                        ogg;