Get input file extension from input file instead of simple

use .aac.

src/builtins/builtins-encoder.cc

 #include <stdio.h>
 #include <iostream>
+#include <array>
 #include <cstdio>
 #include <fstream>
+#include <algorithm>
 
 #include <dlfcn.h>
 #include "builtins-encoder.h"
@@ -371,9 +373,30 @@ BUILTIN(AddAudioURL) {
         isolate, url_str, Object::ToString(isolate, url));
     std::string url_cppstr = url_str->ToCString().get();
 
-
-    std::string outpath = std::tmpnam(nullptr);
-    outpath += ".aac";
+    // Get extension of input file
+    std::string inputExt {}, outpath = std::tmpnam(nullptr);
+    std::array<char, 1> search_for = { '.' };
+    std::array<char, 1> search_for_slash = { '/' };
+
+    auto iter = std::find_end(
+        std::begin(url_cppstr), std::end(url_cppstr),
+        std::begin(search_for), std::end(search_for));
+    auto iter_slash = std::find_end(
+        std::begin(url_cppstr), std::end(url_cppstr),
+        std::begin(search_for_slash), std::end(search_for_slash));
+
+    // If no extension found then leave input file empty
+    // extension. It's format and codec_type will determine by
+    // decoder.
+
+    // FIXME: Condistion still too weak
+    if (iter != std::end(url_cppstr)
+        && iter_slash != std::end(url_cppstr)
+        && iter > iter_slash) {
+
+        inputExt = url_cppstr.substr(std::distance(std::begin(url_cppstr), iter));
+        outpath += inputExt;
+    }
 
     // Add audio file into AudioEffecter
     Handle<Object> volume = args.atOrUndefined(isolate, 2);
@@ -422,6 +445,7 @@ BUILTIN(AddAudioURL) {
     int errorCode = fetchRe(resourcer, url_cppstr.c_str(), outpath.c_str());
     if (errorCode == 200) {
         bool isLoop_bool = isLoop_ == 1;
+        // FIXME: Check success of add operation of audio file is required, at least warning.
         addInputFileAE(audioEffecter, outpath.c_str(), volume_, strong_end_time_, start_time_,
                        end_time_, delay_, duration_, fade_out_duration_, isLoop_bool);
     }