Hi Leif,
I'll check with the dev team to see if this is something they can update - it's tricky because the asset file might have special characters, or not be unique when added to SmartBuilder.
A quick workaround would be to point to the asset you want. Either hosted elsewhere (point directly to a URL - see image1), or toss the file into the directory after publishing (see image2). Just make sure your filename is websafe.
If option 2 is better, and you are publishing frequently, or need this file as part of many packages, there's a way to bundle things with ALL lessons you publish, but that method might be overkill for this situation.
Hope that helps! Let us know how it goes!
- Nav