by Brian LaLonde.
I wanted to use mp4 videos and I found that the Quicktime player would be the default brought up by Moodle 2.5.2.
I found that I had to disable the Quicktime player and leave the HTML 5 Player enabled.
The player checkboxes are listed here:
Site Admin : Appearance : Media Embedding (/admin/settings.php?section=mediasettings)
The Quicktime player is listed as playing Files with extension *.mov, *.mp4, *.m4a, *.mp4 and *.mpg
The HTML 5 player is listed as playing Video files with extension *.webm, *.m4v, *.ogv, *.mp4 and others.
I hope this helps someone else! Cheers!