The DocType attribute is not user-creatable, and there are complications with enabling that, since it isn't obvious what the value should be for various circumstances, there are other attributes that may need to be set based on the actual content, and various other things that would be missing if done manually -- parsing of the page content for searchable text, assigning an item icon from the web site or file, etc.
You can still use Item | Open Document (Ctrl+J) to open externally in the associated application.
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