How can we log file download successes and failures from IIS7?

We have a dedicated server:

• IIS7
• Windows Server 2012 R2 Standard

This servers sole use is to serve files ~120MB in size.

We're looking for the simplest way to log how many downloads were:

• Started
• Completed
• Not completed

For each of the files we host.

What's the best way to go about this?