Project ZIP files are missing additional sources

[urish]

Steps to reproduce

1. Go to a project page that contains multiple source files (e.g. alarm-clock).
2. Go to the "Source code" section, then click on the "Download project" button.
3. Extract the downloaded ZIP file. You will notice that it only contains the main sketch file (e.g. alarm-clock.ino file), but the .h and .cpp files are missing.