Try using s:formattedText for Wiki-style text or otherwise use
<rich:insert src="../yourFile" highlight="yourLanguage" />
and have your text also formatted if necessary. See the Seam and Rich:faces docs for more information.
ui:include should work just fine. If it doesnt work its probably something wrong in your code. Try also renaming the file to xhtml and include that which only contains text. Should be a valid workaround for you.
Both methods work, but <ui:include> specifically expects a structured document, while I don't want/need any tags in my import. so <rich:insert> is a better solution for this case.