public class NewCommand
    {
        public int Command_id { get; set; }
        [FileSize(MaxContentLength = 1 * 1024 * 1024)]
        public HttpPostedFileBase File { get; set; }
    }

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    public sealed class FileSizeAttribute : DataTypeAttribute, IClientValidatable
    {
        private int _maxContentLength = 1 * 1024 * 1024;
        public FileSizeAttribute()
            : base("upload")
        {
            ErrorMessage = string.Format("File is too large, maximum allowed is: {0} MB", MaxContentLength * 1.0 / (1024 * 1024));
        }

        public int MaxContentLength
        {
            get
            {
                return _maxContentLength;
            }
            set
            {
                _maxContentLength = value;
            }
        }

        public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
        {
            var rule = new ModelClientValidationRule
            {
                ValidationType = "filesize",
                ErrorMessage = FormatErrorMessage(metadata.GetDisplayName())
            };

            rule.ValidationParameters["max"] = _maxContentLength;

            yield return rule;

        }

        public override bool IsValid(object value)
        {
            if (value == null)
            {
                return true;
            }

            HttpPostedFileBase valueAsFileBase = value as HttpPostedFileBase;
            if (valueAsFileBase.ContentLength > MaxContentLength)
            {
                ErrorMessage = string.Format("File is too large, maximum allowed is: {0} MB", MaxContentLength * 1.0 / (1024 * 1024));
            }

            return false;
        }
    }

<script type="text/javascript">
    (function ($) {
        $.validator.addMethod("filesize", function (value, element, params) {
            return 100 > 99;
        });

        $.validator.unobtrusive.adapters.add("filesize", ["max"], function (options) {
            options.rules['filesize'] = options.params.max;
            options.messages['filesize'] = options.message;
        });

    } (jQuery));
</script>

        <p id="pUpload" style="display:block;">
            @Html.LabelFor(model => Model.New.File, @Html.Resource("Strings, MaxFileSize"))

            @Html.TextBoxFor(model => Model.New.File, new { type = "file", @style = "border: solid 1px black; font: bold 12px Verdana; width:95%" })
            @Html.ValidationMessageFor(model => Model.New.File)
	</p>