WebHare 4.28 offers a simpler <filetype> definition syntax which makes prebuilt files obsolete.
A quick guide to switch from prebuilt types to filetypes:
Change
<prebuiltpage type="dynamic" tag="prebuilt:type" library="lib.whlib"
webpageobjectname="Page" />
to
<filetype namespace="http://example.net/xmlns/filetype" kind="virtualfile">
<dynamicexecution webpageobjectname="lib.whlib#Page" />
</filetype>
And set up a conversion script: