Image demuxer does not assign a duration to progressive JPEG files

I sometimes create playlists consisting of a mix of images and video files. I always end up having to convert progressive JPEGs to regular JPEG files, because they don't get a duration in the playlist (based on number of seconds specified in the Image demuxer settings).

Tested with 2.2.4 on Windows. I've also seen the same behavior on OS X (also with a 2.2.x version).

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