Thanks so much to the littleBits team for looking into this issue!
Just to offer a little more information for discussion -- The type of files that I need to add as "Additional Files" are MP3, INO (Arduino sketch) and .h (Arduino sketch include) files.
@@Frankje, I haven't tried to upload a ZIP file (yet), but ZIP is certainly a good general file type to support. We're both working on large coding projects where the source code is split into multiple text files (.ino and .h) and both projects produce audio demos (MP3).
All the best to everyone and thank you for listening!