Related: tdf#114428 filter: associate .xhtml with HTML import

Since there is no XHTML import; and this way the expensive "deep"
detection (that calls into all the DLP libs, etc) can be avoided.

Times for a hello world input: 56 -> 23 ms is spent in
Desktop::loadComponentFromURL() (41% of original).

Change-Id: Ia2dec3837cf0c548ae2c5a0ca4d47a57a6cbb92a
Reviewed-on: https://gerrit.libreoffice.org/46387Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

filter/source/config/fragments/types/generic_HTML.xcu

@@ -18,7 +18,7 @@
     <node oor:name="generic_HTML" oor:op="replace" >
         <prop oor:name="DetectService"><value>com.sun.star.comp.filters.PlainTextFilterDetect</value></prop>
         <prop oor:name="URLPattern"><value>private:factory/swriter/web*</value></prop>
-        <prop oor:name="Extensions"><value>html htm</value></prop>
+        <prop oor:name="Extensions"><value>html htm xhtml</value></prop>
         <prop oor:name="MediaType"><value>text/html</value></prop>
         <prop oor:name="Preferred"><value>false</value></prop>
         <prop oor:name="PreferredFilter"><value>HTML</value></prop>