feat: send messages with attachments

close #3

src/structures/MessageMedia.js

@@ -0,0 +1,31 @@
+'use strict';
+
+/**
+ * Media attached to a message
+ * @param {string} mimetype MIME type of the attachment
+ * @param {string} data Base64-encoded data of the file
+ * @param {?string} filename Document file name
+ */
+class MessageMedia {
+    constructor(mimetype, data, filename) {
+        /**
+         * MIME type of the attachment
+         * @type {string}
+         */
+        this.mimetype = mimetype;
+
+        /**
+         * Base64 encoded data that represents the file
+         * @type {string}
+         */
+        this.data = data;
+
+        /**
+         * Name of the file (for documents)
+         * @type {?string}
+         */
+        this.filename = filename;
+    }
+}
+
+module.exports = MessageMedia;