fix(slack): override video/* MIME to audio/* for voice messages (#14941)

* fix(slack): override video/* MIME to audio/* for voice messages

* fix(slack): preserve overridden MIME in return value

* test(slack): fix media monitor MIME mock wiring

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Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/slack/types.ts

@@ -2,6 +2,7 @@ export type SlackFile = {
   id?: string;
   name?: string;
   mimetype?: string;
+  subtype?: string;
   size?: number;
   url_private?: string;
   url_private_download?: string;