My situation is that in one of my services I provide many REST GET APIs to allow the downloading of files (typically xlsx).

In a new API I have a POST API which does exactly the same thing as another GET API (I've copied pasted the code line by line), except that it's using a POST instead of a GET

export async function postDownloadMyFile (
  req: Request,
  res: Response,
  next: NextFunction
): Promise<Response> {

  ... // xlsx workBook creation code
  const buffer =  await workBook.xlsx.writeBuffer();

  res.setHeader('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
  res.setHeader('Content-Disposition', 'attachment; filename=' + 'MyFile.xlsx');
  res.write(buffer, 'binary');
  res.end();

  return res;
}

but when the frontend client makes a request to download the file, the file comes back about twice the size than if I used a GET request and cannot be opened (presumably wrong format/buffer written out wrong).

changing the frontend and backend to use a GET (not modifying the body of the function above) 'fixes' the issue.

Are there some additional headers I'm meant to be setting?

Thanks