Does anyone know of an extension that forces the user to give their email address before they can download an attachment (or emails them a link)?

I think you need a release/ subscription extension which handles downloads as a whole.
There are several available at the JED which support K2 as well.
I think K2 Store also offers digital downloads.

Hello Levi,

You can use OS Downloads from Alledia. The extension has a 5-STAR rating on the JED.
extensions.joomla.org/extension/directory-a-documentation/downloads/osdownloads

It also offers other useful functionality:

- MailChimp integration
- Add a before and after description
- Add an Agreement or Terms of Service

I hope this helps,

Joe Campbell