Fluent NHibernate, varbinary(max) and SQLite

I have a varbinary field in my sql server database that needs to be varbinary(max). I create my database with NHibernate and I use Fluent Nhibernate for my mappings.

I also use SQLite for my unit tests, I use the same mappings I just change the configuration before creating the database in memory.

I get the following problem.

I created this extension method:

 public static IProperty WithMaxVarBinaryLength(this IProperty propertyMap)
 {
     return propertyMap.CustomSqlTypeIs("varbinary(max)");
 }

It works fine on my website, the database is created with a varbinary(max) field, but when I run my unit tests I get the following exception

System.Data.SQLite.SQLiteException: SQLite error near "max": syntax error

Then I found in another question on stackoverflow that we can do this to create a varbinary(max):

public static IProperty WithMaxLength(this IProperty propertyMap)
{
    return propertyMap.WithLengthOf(1000);
}

But I get this exception:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: Content is not a string. at FluentNHibernate.Mapping.PropertyMap.WithLengthOf(Int32 length) in d:\Builds\FluentNH\src\FluentNHibernate\Mapping\PropertyMap.cs:line 166

For the moment I am out of idea, I don't want to have to create manually all my database scripts and I want to continue using SQLite for my unit tests.

Thanks for the help.

By the way, here's my complete mapping, note that I used my extension methods.

public class AttachmentFileMap : ClassMap<AttachmentFile>
{
    public AttachmentFileMap()
    {
        WithTable("AttachmentFiles");

        Id(x => x.Id).GeneratedBy.Identity();
        Map(x => x.Content).WithMaxVarBinaryLength();
        Map(x => x.ContentType);