public IActionResult Index(HomeViewModel model) { if (ModelState.IsValid) { if (!Path.GetExtension(model.Report.FileName).Contains("csv")) { ModelState.AddModelError(nameof(model.Report), "Please upload a file in CSV format."); } else { model.Invoices = _reportHandler .ProcessReport(model.Report, out string message) .ToInvoices(); if (!string.IsNullOrWhiteSpace(message)) { ModelState.AddModelError(nameof(model.Report), message); } } } return(View(model)); }