Hi Vinod,
There's no ultimate solution to this, but you could apply some heuristics to guess the file type. This is what the Unix file program does and it basically uses the so-called [magic numbers|http://en.wikipedia.org/wiki/File_format#Magic_number] (there's also Windows ports out there). For most files this means simply looking at the first few bytes. E.g. a PDF file should always start with a %PDF (first 4 bytes of the file). Similarly for Microsoft Word documents you can also find the bytes, best start is [Gary Kessler's file signature table|http://www.garykessler.net/library/file_sigs.html].
Keep in mind though that this is just an indicator. I.e. of course I could have a text file which starts with %PDF and it's still not a PDF document. However, such logic should still be good enough to filter out Windows executables (<em>.exe</em>) and others.
Anyhow, just check the first couple of bytes of your XSTRING for the magic numbers that you want to allow and reject anything else...
Cheers, harald
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.