I’m not sure why you’re messing with files.
You’ve done the hard part, getting the raw data into openHAB.
Manipulating it to extract a value can be done directly in rules, or via a transformation. If you can’t make XPATH work with it, the whole of javascript is available too.
EDIT - the XPATH trouble is about namespace.
The returned webpage includes
<html xmlns="http://www.w3.org/1999/xhtml" lang="de">
so all the enclosed html tags are in that namespace.
I had a play in a rule; XPath with
"/html/head/title/text()"
returns nothing, because it searches for no-namespace tags.
"/*[name()='html']/*[name()='head']/*[name()='title']/text()"
returns Tirol ZAMG from the
<title>Tirol — ZAMG</title>
section, using namespace.
namespace is discussed in the transformation doc
but I find it hard to follow.