C# LINQ select properties with specific attribute value

I'm looking for a way to select properties that have specific customattributes with specific values in a single LINQ Statement.

I got the properties that have the attribute that I want, but I have no clue how to select it's specific value.

<AttributeUsage(AttributeTargets.Property)>
Public Class PropertyIsMailHeaderParamAttribute
    Inherits System.Attribute

    Public Property HeaderAttribute As String = Nothing
    Public Property Type As ParamType = Nothing

    Public Sub New()

    End Sub

    Public Sub New(ByVal headerAttribute As String, ByVal type As ParamType)
        Me.HeaderAttribute = headerAttribute
        Me.Type = type
    End Sub

    Public Enum ParamType
        base = 1
        open
        closed
    End Enum
    End Class


    private MsgData setBaseProperties(MimeMessage mailItem, string fileName)
    {
        var msgData = new MsgData();
        Type type = msgData.GetType();
        var props = from p in this.GetType().GetProperties()
                    let attr = p.GetCustomAttributes(typeof(Business.IT._21c.AddonFW.PropertyIsMailHeaderAttribute), true)
                    where attr.Length == 1
                    select new { Property = p, Attribute = attr.FirstOrDefault() as Business.IT._21c.AddonFW.PropertyIsMailHeaderAttribute };
    }