Hi,
Attaching files to pdf is a pdf feature, we can't change it. We just implement the attached files section in the pdf spec, it is up to the viewer on how it displays those files. It is designed mostly to attach other kind of files to a pdf, so for example you could have a pdf and embed the xls or csv with the data you used to create that pdf.
Attaching is the only feature of PDF/A3 over PDF/A2: http://en.wikipedia.org/?title=PDF/A#PDF.2FA-3 and as said, it is designed mostly to attach the raw data you used to create the pdf, in case someone wants to process it. I think this is the reason it isn't integrated: If you attach an xlsx file to a pdf file, the xlsx can't open in the pdf reader.
If what you are looking for is to merge existing pdf files, then sadly pdf writer isn't the tool to do that. But if you are creating a new pdf, then you can create an index, and you can embed a png file by calling pdfwriter.DrawImage.