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public IActionResult MyAction([FromServices] IHostingEnvironment environment )
{
  var filePath== System.IO.Path.Combine(environment.WebRootPath,  fileName);
  string contentType;
  new FileExtensionContentTypeProvider().TryGetContentType(filePath, out contentType);
  contentType=contentType ?? "application/octet-stream";
  return File(System.IO.File.ReadAllBytesAsync(filePath),contentType);
}

File(filePath, MimeMapping.GetMimeMapping(filePath),"errorReport.xlsx");