Focus the text input box after successfully attaching a file

Fixes #6284 - does not focus immediately (so as to avoid stealing
 focus) and does not focus if the file browser is cancelled.

ts/state/ducks/composer.ts

@@ -1031,6 +1031,7 @@ export function replaceAttachments(
         attachments: attachments.map(resolveDraftAttachmentOnDisk),
       },
     });
+    dispatch(setComposerFocus(conversationId));
   };
 }