There is no difference what file type you are uploading. Probably your video file is large, and you need to adjust some of php settings in your php.ini (or in .htaccess). Check these parameters:
max_execution_time - time in seconds (must be high enough for your file uploading to complete)
post_max_size - maximum size request with POST method, that PHP will process
upload_max_filesize - maximum size limit for uploading file
You can check any of these settings by creating file say info.php and putting there single line:
<?php phpinfo() ?>
And then upload this file to your site and open it in your browser.
You can read more about adjusting php settings for large files upload here: http://www.radinks.com/upload/config.php