If you want to upload a collection of directories and files (e.g. to upload a presentation), please first tar and then gzip the collection. For example 7-zip (http://www.7-zip.org/) is an open source archiver/unarchiver that has versions for Windows, Linux and Mac. With 7-zip archiving is an easy two step process. First select and tar your files and then gzip the resulting tar file.
To upload a file, click Add Entry, Choose your file, fill in the File Description and fill in the text box with information on using the file. Then click Save and view.