wp-includes/functions.php - function wp_check_filetype()
adding to the $mimes array:
'flv' => 'video/x-flv'
because i want to upload flv videos via the upload form.
if your video is too big for your php-configuration, you might increase the upload_max_filesize by resetting that value. you can do this by placing an .htaccess file into wp-admin folder, which holds the line "php_value upload_max_filesize SIZE", whereas SIZE must be an integer referencing the amount of bytes.
for instance, if you want to reset the maximum file size to 32mb, you have to replace SIZE with 33554432 (1024 * 1024 * 32)...