Use FIRMWARE_FORMAT for "Checking file size" message (qmk#8121)

message.mk

@@ -78,7 +78,7 @@ define GENERATE_MSG_AVAILABLE_KEYMAPS
 endef
 MSG_AVAILABLE_KEYMAPS = $(eval $(call GENERATE_MSG_AVAILABLE_KEYMAPS))$(MSG_AVAILABLE_KEYMAPS_ACTUAL)
 
-MSG_CHECK_FILESIZE = Checking file size of $(TARGET).hex
+MSG_CHECK_FILESIZE = Checking file size of $(TARGET).$(FIRMWARE_FORMAT)
 MSG_FILE_TOO_BIG = $(ERROR_COLOR)The firmware is too large!$(NO_COLOR) $(CURRENT_SIZE)/$(MAX_SIZE) ($(OVER_SIZE) bytes over)\n
 MSG_FILE_TOO_SMALL = The firmware is too small! $(CURRENT_SIZE)/$(MAX_SIZE)\n
 MSG_FILE_JUST_RIGHT = The firmware size is fine - $(CURRENT_SIZE)/$(MAX_SIZE) ($(PERCENT_SIZE)%%, $(FREE_SIZE) bytes free)\n