SqlDataMap.cs

using System;
using System.Data;
using Sitecore.Data;
using Sitecore.Data.Items;

namespace Sitecore.SharedSource.DataImporter.Providers
{
    public class CustomSqlDataMap : Sitecore.SharedSource.DataImporter.Providers.SqlDataMap
    {
        public CustomSqlDataMap(Database db, string connectionString, Item importItem)
            : base(db, connectionString, importItem)
        {}

        protected override string GetFieldValue(object importRow, string fieldName)
        {
            var item = importRow as DataRow;

            var f = item[fieldName];
            
            // it was the missing area because the default solution calls the .ToString()
            // and if it's byte[] then I get "System.Byte[]"
            var bytes = f as byte[];
            if (bytes != null)
            {
                return Convert.ToBase64String(bytes);
            }

            return (f != null) ? f.ToString() : string.Empty;
        }
    }
}