I'm guessing $contentowner is the one that's not working properly. Run the code below with your form and post the output (I commented out the mySQL bit as we don't need that at the minute). I added a print_r statement that prints an array out in a readable way. You can use it to determine which part of the username array you want to use.
<?php
$contenttype = $_POST['contenttype'];
$contentid = $_POST['contentid'];
$contentname = $_POST['contentname'];
$contentowner = $_POST['username'];
// I added this.
print_r($contentowner);
/*
$insert = 'INSERT INTO content (ContentOwner, ContentID, ContentType, ContentName) VALUES ("'.$contentowner.'", "'.$contentid.'", "'.$contenttype.'", "'.$contentname.'")';
mysql_query($insert) or die(mysql_error());
*/
?>