fix: set correct Content-Type for RSS feed audio files

Express's mime package does not recognize .m4b, causing it to fall back
to application/octet-stream. This reuses the existing
getAudioMimeTypeFromExtname utility (already applied to the download
endpoint) to set the correct audio/mp4 header before sendFile.

Fixes #5041

server/managers/RssFeedManager.js

@@ -2,6 +2,7 @@ const { Request, Response } = require('express')
 const Path = require('path')
 
 const Logger = require('../Logger')
+const { getAudioMimeTypeFromExtname } = require('../utils/fileUtils')
 const SocketAuthority = require('../SocketAuthority')
 const Database = require('../Database')
 
@@ -216,6 +217,11 @@ class RssFeedManager {
       res.sendStatus(404)
       return
     }
+    // Express does not set the correct mimetype for m4b files so use our defined mimetypes if available
+    const audioMimeType = getAudioMimeTypeFromExtname(Path.extname(episodePath))
+    if (audioMimeType) {
+      res.setHeader('Content-Type', audioMimeType)
+    }
     res.sendFile(episodePath)
   }