Custom Edit for Sharepoint document library

I have a document library with 2 different Content Types. I need to create custom edit form for editing document properties, I need different custom edit forms for content types.. So I followed the instructions SPD support site to create the edit form using Sharepoint designer. I have done this of several lists and it all works perfectly.

For the document library when I try to attach the custom form to t开发者_JAVA百科he library I don't see the "Content type specific forms" drop down to choose the content type to attach the custom edit form.

How can this be done?

I'm using MOSS 2007

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If using 3rd party components is an option, you can use PowerForms to easily customize the edit forms for each content type without using SP designer