This article explains why you may not be able to create or add text files or folders or see the relevant buttons, because of restrictions set by the group moderator.

Moderators can restrict the "Files" area to themselves or turn off file uploading on a member-by-member basis. If the moderator has turned off your access, you won’t see the Create Folder, Create Text File or Add File buttons in the "Files" area.

You can't upload a file by sending it as an email attachment to the group. You must use your browser to access the "Files" area.