Hi Joel.

Where do you want to add this validation? I make a custom validation control inherited from normal fileupload. The control can be used in a Form.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.IO;
using Umbraco.Forms.Core;
using System.Collections.Specialized;
using System.Configuration;

namespace YourNameSpace

{

public class CustomFileUpload : Umbraco.Forms.Core.Providers.FieldTypes.FileUpload
{
    private int _maxFileSize = 5242880;
    private string _ContentType = "application/pdf";

    public CustomFileUpload()
    {
        Id = new Guid("d8d89001-57ce-4b52-9005-c0c84f6651cb");
        Name = "Custom file upload";
        Description = "Limita a subir solo PDF o JPG y de hasta 1 MB";
        Icon = "icon-cloud-upload ";
        DataType = FieldDataType.String;
        FieldTypeViewName = "FieldType.CustomFileUpload.cshtml";
    }


    NameValueCollection coleccion = (NameValueCollection)ConfigurationManager.GetSection("customfileUploadSettings/tipoCustomfileUploadSettings");
    public override IEnumerable<string> ValidateField(Form form, Field field, IEnumerable<object> postedValues, HttpContextBase context)
    {
        IList<HttpPostedFileBase> files = context.Request.Files.GetMultiple(field.Id.ToString());
        foreach (HttpPostedFileBase file in files)
        {
            int cantidad = 0;
            int.TryParse(coleccion.Get("MaxFileSize"), out cantidad);                

            if (file.ContentLength > cantidad)
            {
                return new[]
                {coleccion.Get("MaxFileSizeMsg")};
            }
            else
            {
                if (file.ContentType != coleccion.Get("ContentTypePDF"))
                {
                    if (file.ContentType == coleccion.Get("ContentTypeIMG"))
                    {
                        string ext = Path.GetExtension(file.FileName).ToUpper();
                        if (ext != coleccion.Get("AllowExtension"))
                        {
                            return new[]
                                    {  coleccion.Get("ContentTypeErroMsg")};
                        }
                    }
                    else
                    {
                        return new[]
                        { coleccion.Get("ContentTypeErroMsg")};
                    }
                }
            }

            if (file.ContentLength == 0)
            { return Enumerable.Empty<string>(); }

        }
        return Enumerable.Empty<string>();
    }
}

}

Gracias Martin.

I wanted to add the simple validation as shown below.

Whilst your method looks good, I need it to work with the Image Cropper datatype. Sorry, I should have been more specific, I didn't mean the file upload datatype.

is working on a reply...