If you are an iPad/iPhone/iPod user, you won't be able to upload files from your device directly to a Moodle assignment. The upload button is greyed out. Never fear, there is a solution!
Make sure you upload your file to your web-based storage solution, SkyDrive Pro (you can also use others like DropBox or Google Drive). That way you'll be able to pull the file from the web-based respository and insert it into the assignment. Stop by the Middle School tech office if you'd like a demonstration.