|  | @@ -203,6 +203,7 @@ class YoutubeDL(object):
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				|  |  |  
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				|  |  |                         Progress hooks are guaranteed to be called at least once
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				|  |  |                         (with status "finished") if the download is successful.
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				|  |  | +    merge_output_format: Extension to use when merging formats.
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				|  |  |  
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				|  |  |  
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				|  |  |      The following parameters are not used by YoutubeDL itself, they are used by
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				|  | @@ -909,6 +910,10 @@ class YoutubeDL(object):
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				|  |  |                                                    'contain the video, try using '
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				|  |  |                                                    '"-f %s+%s"' % (format_2, format_1))
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				|  |  |                                  return
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				|  |  | +                            output_ext = (
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				|  |  | +                                formats_info[0]['ext']
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				|  |  | +                                if self.params.get('merge_output_format') is None
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				|  |  | +                                else self.params['merge_output_format'])
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				|  |  |                              selected_format = {
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				|  |  |                                  'requested_formats': formats_info,
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				|  |  |                                  'format': rf,
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				|  | @@ -921,6 +926,7 @@ class YoutubeDL(object):
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				|  |  |                                  'vbr': formats_info[0].get('vbr'),
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				|  |  |                                  'acodec': formats_info[1].get('acodec'),
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				|  |  |                                  'abr': formats_info[1].get('abr'),
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				|  |  | +                                'ext': output_ext,
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				|  |  |                              }
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				|  |  |                          else:
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				|  |  |                              selected_format = None
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