You should just pass an argument to ApplyXSLTransformation that will set the current filename. Example:

ApplyXSLTransformation(savepath);

function ApplyXSLTransformation(string savepath)
...
...
end function

Within the ApplyXSLTransformation function, replace all "fileupload1.filename" with savepath.

If you look at it, you are creating a new file name with the .html end tag, then saving it.

Now in the function you are trying to grab the supplied filename, which still has the .xml, so it doesn't exist. This is why you get the error.

You need to pass the new file name to the function, which is "savepath". Figure out a way to pass that to the function in order to grab the correct file.