How do I correct my script, that it doesn't matter what file name has the .xml that is uploaded?

My ruby script edits a .xml file as follows:

stringComplete = File.read("TheUpload.xml",encoding: "UTF-8").gsub(my mystery gsub)

and generate it back like this:

newfile = File.open("EditXML.xml", "w") 
newfile.puts "method(xyz)"

How do I correct my script, that it doesn't matter what file name has the .xml that is uploaded and that every single user gets his own edited .xml file and not by someone else?