add file message kind

17.md

@@ -35,6 +35,35 @@ Tags that mention, quote and assemble threading structures MUST follow [NIP-10](
 
 Kind `14`s MUST never be signed. If it is signed, the message might leak to relays and become **fully public**.
 
+## File Message Kind
+
+```jsonc
+{
+  "id": "<usual hash>",
+  "pubkey": "<sender-pubkey>",
+  "created_at": "<current-time>",
+  "kind": 15,
+  "tags": [
+    ["p", "<receiver-1-pubkey>", "<relay-url>"],
+    ["p", "<receiver-2-pubkey>", "<relay-url>"],
+    ["e", "<kind-14-id>", "<relay-url>", "reply"], // if this is a reply
+    ["subject", "<conversation-title>"],
+    ["file-type", "<file-mime-type>"],
+    ["encryption-algorithm", "<encryption-algorithm>"],
+    ["decryption-key", "<decryption-key>"]
+    // rest of tags...
+  ],
+  "content": "<file-url-or-base64-content-in-plain-text>"
+}
+```
+
+Kind 15 is used for sending encrypted file event messages:
+
+- `file-type`: Specifies the MIME type of the attached file (e.g., `image/jpeg`, `audio/mpeg`, etc.).
+- `encryption-algorithm`: Indicates the encryption algorithm used for encrypting the file. Supported algorithms may include `aes-gcm`, `chacha20-poly1305`,`aes-cbc` etc.
+- `decryption-key`: The decryption key that will be used by the recipient to decrypt the file.
+- `content`: Contains either the URL of the file (`<file-url>`) or the encrypted data in base64 format (`<base64-content-in-plain-text>`).
+
 ## Chat Rooms
 
 The set of `pubkey` + `p` tags defines a chat room. If a new `p` tag is added or a current one is removed, a new room is created with clean message history.