Ftid!.txt
Download File ===> https://shoxet.com/2tlyI9
Note the above instructions assume you have Tools > Clock > 16 MHz external selected. If you are using a different clock selection, you only need to change the upload.port property in boards.txt associated with that configuration.
If you wanted, you could use only the bootloader file from MegaCore and go back to using the Tools > Board > Arduino Mega menu selection with your board. The only disadvantage of that is the optiboot bootloader used by MegaCore fits in the ATmega2560's minimum 1 kB boot section, which the outdated STK500 bootloader used by Arduino AVR Boards for the Arduino Mega requires an 8 kB boot section! So the Arduino Mega board selection will tell you you're out of program memory when you actually still have 7 kB free. The ATmega2560 has so much flash memory that's not likely to be a problem. Of course fixing that only requires changing one line in boards.txt. 59ce067264