Catching directory transversals passed through QueryString

Ok, the gist of the problem is I have a site my predecessor created
that much of it goes through a single asp.net app. For some reason,
instead of using templates or includes, he passed everything through
this one bit of code that dumped html and flat text in the middle of
the page. To keep it dynamic, he has the querystring take in a
filename. eg.
http://www.oursite.net/default.aspx?filename=/folder/file.txt

Of course, this leaves it wide open for
http://www.oursite.net/default.aspx?filename=../web.config
or
http://www.oursite.net/default.aspx?filename=../../../Inetpub/wwwroot/default.aspx
or worse.

For now, I'm just using regex to require either ".txt" or ".htm?" and
then catch "../" and hit the breaks if it finds that. I'm concerned,
though, that there might be other ways of exploiting this.

.

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