Hi daviiie; I have exactly the same issue and tried this, thinking SaveAs locking the files as you did but id didn't work, so something else is locking the files:

// Request.Files[i].SaveAs(fileName_save);

using (FileStream fs = new FileStream(fileName_save, FileMode.Create))
{
byte[] bytes = new byte[Request.Files[i].InputStream.Length];
Request.Files[i].InputStream.Read(bytes, 0, (int)Request.Files[i].InputStream.Length);
fs.Write(bytes, 0, bytes.Length);
fs.Close();
}

Did you have any progress for the issue? Thanks.