Implements attachment uploading

kordophone/src/tests/test_client.rs

@@ -127,4 +127,15 @@ impl APIInterface for TestClient {
     ) -> Result<Self::ResponseStream, Self::Error> {
         Ok(futures_util::stream::iter(vec![Ok(Bytes::from_static(b"test"))]).boxed())
     }
+
+    async fn upload_attachment<R>(
+        &mut self,
+        data: tokio::io::BufReader<R>,
+        filename: &str,
+    ) -> Result<String, Self::Error>
+    where
+        R: tokio::io::AsyncRead + Unpin + Send + Sync + 'static,
+    {
+        Ok(String::from("test"))
+    }
 }