Using a custom collection for encapsulation

I want to create my own collection which is inherited from ReadOnlyCollection in order to encapsulate my domain model logic in it.

public class Product : Entity
{
    public string Name { get; set; }

    private readonly List<ProductAttachment> _attachments = 
        new List<ProductAttachment>();

    public Attachment<ProductAttachment> Attachments => new Attachment<ProductAttachment>(_attachments);
}

public class Attachment<TFileEntity> : ReadOnlyCollection<TFileEntity> where TFileEntity : FileEntity<int>
{
    public void AddAttachment(TFileEntity fileEntity)
    {
        Items.Add(fileEntity);
    }

    public Attachment(IList<TFileEntity> list) : base(list)
    {
    }
}

Next, I try to add an attachment

foreach (var attachment in request.Attachments)
{
    product.Attachments.AddAttachment(new ProductAttachment(attachment.FullName, attachment.Content))    
}

And after I try to add the object via _context.Products.Add(product) or get, I get an error

No coercion operator is defined between types 'System.Collections.Generic.List1[Domain.Entities.ProductAttachment]' and 'Domain.Common.Attachment1[Domain.Entities.ProductAttachment]'.

How can I solve this problem? I would like to end up with a custom collection where the logic is encapsulated. Maybe I'm doing something wrong?