I would assume you need to convert the file first into a word document, assuming it is a Lotus file. The file format is not natively supported by iOS, but perhaps someone else knows of an app, which can open it directly.
Can filer open .lwp files? Because if not, it won't appear in the Open In... list from Mail. And as far as I know is filer only a file manager with zip/unzip capabilities, so I wouldn't expect it to open .lwp files.
I realize, that this doesn't help much, because you want to just press on the attachment and have it open in whatever app can read the file type. However, I don't think this direct approach is possible, atm.