Merge some file writing extension checks from OpenJK.

Thanks Ensiform.
https://github.com/JACoders/OpenJK/commit/05928a57f9e4aae15a3bd0
https://github.com/JACoders/OpenJK/commit/ef124fd0fc48af164581176
void Con_MessageMode2_f (void) {
	chat_playerNum = -1;
	chat_team = qtrue;
	Field_Clear( &chatField );
	chatField.widthInChars = 25;
	Key_SetCatcher( Key_GetCatcher( ) ^ KEYCATCH_MESSAGE );
}

void Con_MessageMode2_f (void) {
	chat_playerNum = -1;
	chat_team = qtrue;
	Field_Clear( &chatField );
	chatField.widthInChars = 25;
	Key_SetCatcher( Key_GetCatcher( ) ^ KEYCATCH_MESSAGE );
}
