Yes. I know most developers avoid this because the browser does not allow such hidden downloads, and ask the user if and where to save the file (Known web security issue).
And also I could not know when the download is over.
But
If my client realy insist that he will be able to click once on a web page an have an outlook window open and filled,containing an attachment. can I do so using my own customized browser?
Using a customized browser, yes. (If your client runs windows and has office installed…)
For example, create a .Net winform-program with a IE-browser-control in it.
Add hook to IE-control to get when user clicks a specific a-link.
Then perform a download of file from winform-program, use office-api/automation/3rdpart-control from winform-program to fire up a new mail and add attachment to the mail.