Allows Cargo borgs to carry letters (#19271)

* The borgs work as mailboxes now

* Actually they can have upgrades, as a treat

* Missing

code/game/objects/items/robot/robot_upgrades.dm

@@ -512,6 +512,36 @@
 	to_chat(R, span_notice("Sheet snatcher upgraded!"))
 	return TRUE
 
+/obj/item/borg/upgrade/restricted/adv_mailbag
+	name = "robotic mailbag capacity module"
+	desc = "Used to expand the mailbag storage."
+	icon_state = "cyborg_upgrade3"
+	item_state = "cyborg_upgrade"
+	module_flags = BORG_MODULE_MINER
+	require_module = TRUE
+
+/obj/item/borg/upgrade/restricted/adv_mailbag/action(mob/user, mob/living/silicon/robot/R)
+	if(..()) return FALSE
+
+	if(!R.supports_upgrade(type))
+		generic_error(user, R, type)
+		return FALSE
+
+	var/obj/target_module = R.has_upgrade_module(/obj/item/storage/bag/mail/borg)
+	if(!target_module)
+		to_chat(user, span_warning("This robot has had its letter compartment removed!"))
+		return FALSE
+
+	if(R.has_restricted_upgrade(type))
+		to_chat(R, span_warning("Letter compartment was already upgraded!"))
+		to_chat(user, span_warning("There's no room for another letter compartment upgrade!"))
+		return FALSE
+
+	var/obj/item/storage/bag/mail/borg/mailbag = target_module
+	mailbag.upgrade()
+	to_chat(R, span_notice("Letter compartment upgraded!"))
+	return TRUE
+
 /*	###############################################
 	# Unsorted section. All cargo modules go here.#
 	###############################################*/

code/game/objects/mail.dm

@@ -378,6 +378,15 @@
 		/obj/item/pen
 	)
 
+/obj/item/storage/bag/mail/borg
+	name = "letter compartment"
+	desc = "A compartment specifically made for small postage."
+
+/obj/item/storage/bag/mail/borg/proc/upgrade()
+	name += " of holding"
+	storage_slots = 45
+	max_storage_space = 75
+
 // Mail Scanner
 /obj/item/mail_scanner
 	name = "mail scanner"