NFT metadata extension tag

CIP-0049/CIP-0049.md

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+---
+CIP: 49?
+Title: 721 Metatdatum extenstion tag
+Authors: Jack ([9000] pool, https://forum.cardano.org/u/jack7e/, @ada9000_ twitter)
+Comments-URI: https://forum.cardano.org/t/cip-extended-721/
+Status: Draft
+Type: Informational
+Created: 2022-09-28
+---
+
+# Abstract
+
+This standard proposes a method to extend the 721 metadatum standard bypassing the restrictions created by the version tag.
+
+# Motivation
+
+- NFT's on Cardano are static and rely on a version tag change to implement new features.
+- The version tag is not very modular and restricts innovation.
+
+# Specification
+
+- The standard 721 metadatum tag is used.
+- The version tag is **deprecated** at it's current `version:2` by the `ext` keyword.
+  - To use the version 1 tag I suggest `"ext":["v1"]`
+  - To use the version 2 tag I suggest `"ext":["v2"]`
+  - This might seem odd at first but going foward instead of adding version 3 we just define the CIP as an optional `ext` array element. For example CIPX with version 2 would be `"ext":["v2","cipx"]`
+- The `ext` tag is added within the `"721"` metadatum tag
+- The `ext` tag should be lowercase. When parsing nft metadata the developer will use a to lowercase function on all elements in the `ext` array
+
+```json
+{
+   "721":{
+      "ext": <array>
+      "<policy_id>":{
+         "<asset_name>": {
+            "name": <string>,
+            "image": <uri | array>
+         }
+      }
+   }
+}
+```
+
+# Example usage
+
+The theoretical cipX in this example defines a script tag that must be present. To implement this CIP in our 721 metadatum JSON we simply add CIPX to the `"ext"` array.
+
+```json
+{
+   "721":{
+      "ext": ["cipX"]
+      "<policy_id>":{
+         "script":"<script_policy_id>"
+         "<asset_name>": {
+            "name": <string>,
+            "image": <uri | array>
+         }
+      }
+   }
+}
+```
+
+Where cipX defines some property for the **JSON**. In this particular example cipX is a fictional CIP that requires the nft to contain a `"script"` tag with a `"<script_policy_id>"`.