Opening a file of a particular type

Hi there

I feel confident that this is possible, and I have searched, but am unable to find how to do this.

Brief:
My database contains blobs which represent attachments that users have attached.  I have a button "open attachment".  This button should then, well, open the attachment using whatever programme is defined in the registry as the default programme to open the attachment.

WHere I have got to:
I can attach the blob. I can then write it out to a temporary directory using the original file name.  I can get the file type from the registry using the extension of the filename I write out.

What I can't figure out how to do:
Lookup the registry and get the associated programme based on the filetype to launch the file.

All help appreciated

Clark