Get preview file name from CustomDownload.partial_filename

Add 'custom_downloader' member var to DownloadTask. Use it in
episode.get_playback_url() to get a preview file name.

Also add episode.can_preview() method and use it in ep.can_play().

src/gpodder/download.py

@@ -717,6 +717,7 @@ class DownloadTask(object):
         self.speed = 0.0
         self.progress = 0.0
         self.error_message = None
+        self.custom_downloader = None
 
         # Have we already shown this task in a notification?
         self._notification_shown = False
@@ -898,6 +899,7 @@ class DownloadTask(object):
             else:
                 downloader = DefaultDownloader.custom_downloader(self._config, self.episode)
 
+            self.custom_downloader = downloader
             headers, real_url = downloader.retrieve_resume(self.tempname, self.status_updated)
 
             new_mimetype = headers.get('content-type', self.__episode.mime_type)

src/gpodder/model.py

@@ -480,7 +480,15 @@ class PodcastEpisode(PodcastModelObject):
         """
         # gPodder.playback_episodes() filters selection with this method.
         """
-        return self.was_downloaded(and_exists=True) or self.can_stream(config)
+        return (self.was_downloaded(and_exists=True)
+                or self.can_preview()
+                or self.can_stream(config))
+
+    def can_preview(self):
+        return (self.downloading
+                and self.download_task.custom_downloader is not None
+                and self.download_task.custom_downloader.partial_filename is not None
+                and os.path.exists(self.download_task.custom_downloader.partial_filename))
 
     def can_stream(self, config):
         """
@@ -620,11 +628,10 @@ class PodcastEpisode(PodcastModelObject):
         Also returns the filename of a partially downloaded file
         in case partial (preview) playback is desired.
         """
-        url = self.local_filename(create=False)
+        if (allow_partial and self.can_preview()):
+            return self.download_task.custom_downloader.partial_filename
 
-        if (allow_partial and url is not None and
-                os.path.exists(url + '.partial')):
-            return url + '.partial'
+        url = self.local_filename(create=False)
 
         if url is None or not os.path.exists(url):
             # FIXME: may custom downloaders provide the real url ?