Develop a function named decode$($message$\_$file$).$ This function should read an encoded message from a $.$txt file and return its decoded version as a string. Your function must be able to process an input file. In this file, each line contains a number followed by a word. The task is to decode a hidden message based on the arrangement of these numbers into a "pyramid" structure. The key to decoding the message is to use the words corresponding to the numbers at the end of each pyramid line $($in this example, $1,3,$ and $6).$ You should ignore all the other words.