Merge some file writing extension checks from OpenJK.

Thanks Ensiform.
https://github.com/JACoders/OpenJK/commit/05928a57f9e4aae15a3bd0
https://github.com/JACoders/OpenJK/commit/ef124fd0fc48af164581176
static void Com_Crash_f( void ) {
	* ( volatile int * ) 0 = 0x12345678;
}

static void Com_Crash_f( void ) {
	* ( volatile int * ) 0 = 0x12345678;
}
