How do I use project folders in a cloud save?

This may be a stupid question: my project file is getting quite large, and I thought saving it as a folder would speed things up when saving to Dropbox.

But when I try to save as a project folder, it only points to my hard drive. And if I copy a project folder into my C3 Dropbox folder, C3 doesn't see the c3proj file.

Am I doing something wrong? Or are project files not supported in cloud saves?

Thanks very much!