How to print any file-type? The app he referenced to seems to open application that is set as default for that file-type, calls print, and then close the app. Anyone knows how to do that?

In the general case, this is not possible. However, you may be able to handle enough of the common file types to make people happy.

Here are just a couple of the obstacles you will run into:

• For some file types, nothing knows how to open the file except the app that created it, which may not be installed. A good example of this is Publisher; nothing else can read its files, and it is usually not installed (even when Office is installed, Publisher frequently is not), but it is common enough that people *will* try to send that format as an email attachment. For that matter, somebody will try to send some hyper-obscure file format that is proprietary to some shareware app nobody ever heard of. In rare cases, the app needed to open the file may not even be available for Windows (e.g., some popular Mac software).
• If the content-type and the extension do not agree, it is not always clear which is correct. A human can use trial and error, because if he tries to open it in the wrong app it'll "look like garbage", and he won't hit print. You may need to employ magic-number checking in these cases. Joy.
• Some things are not, technically speaking, sensible to print, but people will want to anyway. Somebody will get sent a MS-TNEF attachment, a screen saver, or an EXE, and want to print it. This is especially true of TNEF, which frequently does not contain what the message claims it contains. People freqently try to attach digital photographs, believe they have done so, and say in the message what it's a picture of, when in fact the attachment is 20 bytes and contains nothing of the kind.

If it were me, I would ask the guy for an exhaustive list of all the file formats he wants supported, and I'd evaluate that list *before* agreeing to do it. It's easy to handle BMP, PNG, JPEG, and GIF, and it's not hard to pass HTML off to IE, and Office formats off to Office if it's installed. You can look inside ZIP files and see if there are any filetypes you know how to print in there. Plain text of course is no problem, and attached email messages you can handle easily enough (recursively, probably). Find out if that's enough to make him happy.