How to Increase the max upload file size in ASP.NET MVC

By default, you only upload up to 4MB (4096 KB) when hosting your website on IIS. To increase file size when uploading on IIS 7 you need to open the web.config file, then add the configuration as below.

<system.webServer>
    <security>
      <requestFiltering>
        <requestLimits maxAllowedContentLength="52428800" />
      </requestFiltering>
    </security>    
</system.webServer>

As you can see, I have increased the max file upload size to 50MB. You can also increase the maximun upload file size by creating a custom file size attribute as shown below.

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web;
using System.Web.Mvc;

namespace Customs
{
    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
    public class FileSizeAttribute : ValidationAttribute, IClientValidatable
    {
        public int? MaxBytes { get; set; }

        public FileSizeAttribute(int maxBytes)
            : base("Please upload a supported file.")
        {
            MaxBytes = maxBytes;
            if (MaxBytes.HasValue)
                ErrorMessage = $"Please upload a file of less than {MaxBytes.Value} bytes.";
        }

        public override bool IsValid(object value)
        {
            HttpPostedFileBase file = value as HttpPostedFileBase;
            if (file != null)
            {
                bool result = true;
                if (MaxBytes.HasValue)
                    result &= (file.ContentLength < MaxBytes.Value);
                return result;
            }
            return true;
        }

        IEnumerable<ModelClientValidationRule> IClientValidatable.GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
        {
            var rule = new ModelClientValidationRule
            {
                ValidationType = "filesize",
                ErrorMessage = FormatErrorMessage(metadata.DisplayName)
            };
            rule.ValidationParameters["maxbytes"] = MaxBytes;
            yield return rule;
        }
    }
}

Open the FilterConfig.cs file, then add FileSizeAttribute to the RegisterGlobalFilters method that lets you filter the upload file size to IIS.

using System.Web;
using System.Web.Mvc;

namespace CSharpCode
{
    public class FilterConfig
    {
        public static void RegisterGlobalFilters(GlobalFilterCollection filters)
        {
            filters.Add(new FileSizeAttribute());
        }
    }
}