You shouldn't need to convert to text first to use the preprocess option. That said, if it doesn't work then you may need to open a bug with the file.
While that message is a header/footer, it doesn't need to be to create a rule to get rid of it. Can't tell you exactly what the rule will be, you need to look at the source code of your file. There is a little magic wand next to the header or footer regex text-box. Just click it there, then write your pattern. It's probably something like:
but you'll need to make it work for whatever is actually in your file.