public class SomeModel
    {
        [Required, FileExtensions(Extensions = ".csv", ErrorMessage = "Specify a CSV file. (Comma-separated values)")]
        public HttpPostedFileBase File { get; set; }
    }

@using (Html.BeginForm("Some", "Some", FormMethod.Post, new { enctype = "multipart/form-data" }))
        {
            @Html.TextBoxFor(m => m.File, new { type = "file" })
            @Html.ValidationMessageFor(m => m.File)
            <input type="submit" value="upload" />
        }