This is how we're doing our custom field types, can you show the rest of the FieldType code?

public class CareCategoryField : FieldType
{
    [Required(ErrorMessage = ValidationRequiredCareCategory)]
    [Setting("Care category", view = "TextField")]
    public string DefaultValue { get; set; }

    public CareCategoryField()
    {
        Id = new Guid(FieldTypeId.CareCategoryField.Description());
        Name = "Care category";
        FieldTypeViewName = "FieldType.CareCategory.cshtml";
        Description = "Used to capture the mandatory care code";
        Icon = "icon-categories";
        DataType = FieldDataType.String;
        Category = "Simple";
        SortOrder = 10;
    }
    public override string GetDesignView()
    {
        return "/App_Plugins/UmbracoForms/Backoffice/Common/FieldTypes/hiddenfield.html";
    }

    public override bool HideLabel
    {
        get { return true; }
    }

    public override bool StoresData
    {
        get { return true; }
    }
}

I tried to use your settings and it still doesn't save. Here is my code:

public class NewTestField : FieldType
{
    [Setting("Test Setting", view = "TextField")]
    public string TestSetting { get; set; }

    public NewTestField()
    {
        this.Id = new Guid(FieldTypeId.GetNewTestFieldId());
        this.Name = "AgreementsField";
        this.Description = "Render a custom text field.";
        this.Icon = "icon-autofill";
        this.DataType = FieldDataType.String;
        this.SortOrder = 10;
        this.SupportsRegex = true;
    }

    public override string GetDesignView()
    {
        return "/App_Plugins/UmbracoForms/Backoffice/Common/FieldTypes/hiddenfield.html";
    }

    public override bool HideLabel
    {
        get { return true; }
    }

    public override bool StoresData
    {
        get { return true; }
    }

    // You can do custom validation in here which will occur when the form is submitted.
    // Any strings returned will cause the submit to be invalid!
    // Where as returning an empty ienumerable of strings will say that it's okay.
    public override IEnumerable<string> ValidateField(Form form, Field field, IEnumerable<object> postedValues, HttpContextBase context)
    {
        var returnStrings = new List<string>();

        if (!postedValues.Any(value => value.ToString().ToLower().Contains("custom")))
        {
            returnStrings.Add("You need to include 'custom' in the field!");
        }

        // Also validate it against the original default method.
        returnStrings.AddRange(base.ValidateField(form, field, postedValues, context));

        return returnStrings;
    }
}

What version of forms does this work on for you?

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