public IActionResult OnPost(long id, string subject, List<IFormFile> files)
{
    foreach (var formFile in files)
    {
        if (formFile.Length > 0)
        {
            using (var stream = new FileStream(fileFullPath, FileMode.Create))
            {
                formFile.CopyTo(stream);
            }
        }
    }
}

using (FileStream fs = new FileStream(fileSavePath, FileMode.OpenOrCreate, FileAccess.Write))
{
     MemoryStream ms = new MemoryStream();
     Request.Body.CopyTo(ms);
     ms.Position = 0;
     byte[] buffer1 = new byte[ms.Length];
     ms.Read(buffer1, 0, buffer1.Length);
     ms.Dispose();
     fs.Write(buffer1, 0, buffer1.Length);
}