We are aware that there is a limitation in the ability to upload files to your Stash through the Web site. This limitation is being addressed in a future update to the Web Access site.
In the meantime, if you are using the Mozy app on a mobile device, you have the ability to send the file to your Stash. Try emailing the file from public computer to your email address and then download the file to your mobile. You should have an "Open In" option for the file. Click Open In and select Mozy as the application to use when opening the file. This will upload the file to your account.