Ignoring the detail that that won't compile - Regex.Split returns an array of strings, not a single string. It would defeat the purpose somewhat if it didn't, wouldn't it?
I'm not positive what you are trying to split on: it looks like you want to split the string everywhere is contains the text ".xls" which seems silly, but it may be what you want to do, I don't know. If so, then you need to escape the "." as that is a regex character meaning "any character at all":
Get a copy of Expresso
] - it's free, and it examines and generates Regular expressions.