In Zoho Projects, as a part of "Email Collaboration" there is an option to upload files to a project via email, and the files will be uploaded to "Discussions" folder in that project, by default. Option to upload files via email is available at the footer section of the Documents Tab.

Now, you can upload the files to the folder you desired too. Its simple. Go to Documents Tab, click on the folder & you will see an unique email id for that folder at the bottom of the page, send the files to that email id - which will upload the files to that folder.

Prasanna