fixed custom vendor cache audio stores file extension

lib/synth-audio.js

@@ -170,6 +170,8 @@ async function synthAudio(client, createHash, retrieveHash, logger, stats, { acc
     renderForCaching
   });
   let filePath;
+  // used only for custom vendor
+  let fileExtension;
   filePath = makeFilePath({vendor, voice, key, salt, renderForCaching});
   debug(`synth key is ${key}`);
   let cached;
@@ -201,7 +203,17 @@ async function synthAudio(client, createHash, retrieveHash, logger, stats, { acc
     debug('result WAS found in cache');
     servedFromCache = true;
     stats.increment('tts.cache.requests', ['found:yes']);
-    audioBuffer = Buffer.from(cached, 'base64');
+    if (vendor.startsWith('custom')) {
+      // custom vendors support multiple mime types such as: mp3, wav, r8, r16 ...etc,
+      // mime type/file extension is available when http response has header Content-type.
+      // In cache, file extension is store together with audiBuffer in a json.
+      // Normal cache audio will be base64 string
+      const payload = JSON.parse(cached);
+      filePath = filePath.replace(/\.[^\.]*$/g, payload.fileExtension);
+      audioBuffer = Buffer.from(payload.audioBuffer, 'base64');
+    } else {
+      audioBuffer = Buffer.from(cached, 'base64');
+    }
     client.expire(key, EXPIRES).catch((err) => logger.info(err, 'Error setting expires'));
   }
   if (!cached) {
@@ -268,7 +280,7 @@ async function synthAudio(client, createHash, retrieveHash, logger, stats, { acc
           renderForCaching, disableTtsStreaming});
         break;
       case vendor.startsWith('custom') ? vendor : 'cant_match_value':
-        ({ audioBuffer, filePath } = await synthCustomVendor(logger,
+        ({ audioBuffer, filePath, fileExtension } = await synthCustomVendor(logger,
           {credentials, stats, language, voice, text, filePath}));
         break;
       default:
@@ -282,7 +294,14 @@ async function synthAudio(client, createHash, retrieveHash, logger, stats, { acc
     debug(`tts rtt time for ${text.length} chars on ${vendorLabel}: ${rtt}`);
     logger.info(`tts rtt time for ${text.length} chars on ${vendorLabel}: ${rtt}`);
 
-    client.setex(key, EXPIRES, audioBuffer.toString('base64'))
+    const base64Audio = audioBuffer.toString('base64');
+    const cacheContent = vendor.startsWith('custom') ?
+      JSON.stringify({
+        audioBuffer: base64Audio,
+        fileExtension
+      }) : base64Audio;
+
+    client.setex(key, EXPIRES, cacheContent)
       .catch((err) => logger.error(err, `error calling setex on key ${key}`));
   }
 
@@ -729,9 +748,11 @@ const synthCustomVendor = async(logger, {credentials, stats, language, voice, te
     const regex = /\.[^\.]*$/g;
     const mime = response.headers['content-type'];
     const buffer = await response.arrayBuffer();
+    const fileExtension = getFileExtFromMime(mime);
     return {
       audioBuffer: buffer,
-      filePath: filePath.replace(regex, getFileExtFromMime(mime))
+      filePath: filePath.replace(regex, fileExtension),
+      fileExtension
     };
   } catch (err) {
     logger.info({err}, `Vendor ${vendor} returned error`);