Add a way to send messages with attachments.

src/database/entities/autumn.rs

@@ -0,0 +1,21 @@
+use serde::{Serialize, Deserialize};
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+#[serde(tag = "type")]
+enum Metadata {
+    File,
+    Image { width: isize, height: isize },
+    Video { width: isize, height: isize },
+    Audio,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct File {
+    #[serde(rename = "_id")]
+    pub id: String,
+    filename: String,
+    metadata: Metadata,
+    content_type: String,
+
+    message_id: Option<String>
+}