Getting the file in memory is the easy bit! It's just an <input
type="file" tag on an html form.

If you don't know html, that probably doesn't mean much to you. But
imagine a web-based email client, say. There's probably a button called
"Add attachment" which will show you a "File Open" common dialog. You
choose a filename and hit the "Send" button. Behind the scenes, that
file you selected is uploaded to the web server (along with subject,
body fields etc. etc.). So at the server, it has to take the input,
turn it into an email, and take this uploaded file and turn it into an
attachment.

I'm doing the same kind of thing except I'm not building an email
client, I'm building something that will interpret a spreadsheet.

As regards "impressive", it obviously sounds more glamorous than it
actually is!

Pete