diff --git a/youtube_dl/PostProcessor.py b/youtube_dl/PostProcessor.py
index 75b4f2cb5..3bc4e7998 100644
--- a/youtube_dl/PostProcessor.py
+++ b/youtube_dl/PostProcessor.py
@@ -11,6 +11,7 @@ from .utils import (
     PostProcessingError,
     shell_quote,
     subtitles_filename,
+    build_part_filename,
 )
 
 
@@ -518,7 +519,7 @@ class FFmpegJoinVideosPP(FFmpegPostProcessor):
         parts = information.get('parts')
         if parts is None or len(parts) == 1:
             return (True, information)
-        parts_files = [u'%s.%s' % (filename, i) for (i, _) in enumerate(parts)]
+        parts_files = [build_part_filename(filename, i) for (i, _) in enumerate(parts)]
         files_file = u'%s.videos' % filename
         with io.open(encodeFilename(files_file), 'w', encoding='utf-8') as f:
                     for video in parts_files:
@@ -527,7 +528,7 @@ class FFmpegJoinVideosPP(FFmpegPostProcessor):
         try:
             self.run_ffmpeg(files_file, filename, ['-c', 'copy'], ['-f', 'concat'])
         except FFmpegPostProcessorError:
-            return False
+            return False, information
         os.remove(encodeFilename(files_file))
         # We have to manually remove the parts if requested
         if not self._downloader.params.get('keepvideo', False):
diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index 0500bb146..79a21af7f 100644
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -43,6 +43,7 @@ from .utils import (
     SameFileError,
     sanitize_filename,
     subtitles_filename,
+    build_part_filename,
     takewhile_inclusive,
     UnavailableVideoError,
     write_json_file,
@@ -790,7 +791,7 @@ class YoutubeDL(object):
                             for (i, part) in enumerate(parts):
                                 part_info = dict(info_dict)
                                 part_info.update(part)
-                                part_filename = u'%s.%s' % (filename, i)
+                                part_filename = build_part_filename(filename, i)
                                 parts_success.append(self.fd._do_download(part_filename, part_info))
                             success = all(parts_success)
                 except (compat_urllib_error.URLError, compat_http_client.HTTPException, socket.error) as err:
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index 946e90e93..c94d1519f 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -775,6 +775,10 @@ def determine_ext(url, default_ext=u'unknown_video'):
 def subtitles_filename(filename, sub_lang, sub_format):
     return filename.rsplit('.', 1)[0] + u'.' + sub_lang + u'.' + sub_format
 
+def build_part_filename(final_filename, part_index):
+    (name, ext) = os.path.splitext(final_filename)
+    return '%s.%d%s' % (name, part_index, ext)
+
 def date_from_str(date_str):
     """
     Return a datetime object from a string in the format YYYYMMDD or