You can upload all kind of content to notes, tasks and even comments. Every user who has access to a project can see all the content that is attached to any item within that project. If you upload file A to a task A, it will be attached to that task. So if you delete task A, all attachements you uploaded to that task A will also be deleted! So if you attached the same file to another task or note, this note will also lose the attachement. SO always make sure to either don’t delete anything if not necessary or upload important files again for every new task or note.
File size limit
You might run into trouble uploading a large file. This is not a bug or anything – it simply has to do with either your WordPress install or the server you are running the site on. There is an article over at wpmudev’s blog to help you out on this: Increase memory limit. (note: while we check the quality of the sites we link, we are not responsible for their content).