Moves DraftAttachments into redux

ts/util/fileToBytes.ts

@@ -0,0 +1,18 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function fileToBytes(file: Blob): Promise<Uint8Array> {
+  return new Promise((resolve, rejectPromise) => {
+    const FR = new FileReader();
+    FR.onload = () => {
+      if (!FR.result || typeof FR.result === 'string') {
+        rejectPromise(new Error('bytesFromFile: No result!'));
+        return;
+      }
+      resolve(new Uint8Array(FR.result));
+    };
+    FR.onerror = rejectPromise;
+    FR.onabort = rejectPromise;
+    FR.readAsArrayBuffer(file);
+  });
+}