The Movie Extends Header is optional, and provides the overall duration,
including fragments, of a fragmented movie. If this box is not present, the
overall duration must be computed by examining each fragment.
author  Alexander Simm

The fragment duration is an integer that declares length of the
presentation of the whole movie including fragments (in the timescale
indicated in the Movie Header Box). The value of this field corresponds
to the duration of the longest track, including movie fragments. If an
MP4 file is created in real-time, such as used in live streaming, it is
not likely that the fragment duration is known in advance and this box
may be omitted.
return  the fragment duration