The file management section shows you all the documents you have uploaded (for example, when linking to a file via a button in any of the modules with button functions). Here you can see when you uploaded the file and how big it is. The latter can be useful when trying to free up space in your account, should you have been hitting account size limits due to the number / size of files you have uploaded.
As a rule of thumb, we recommend files be no bigger than 512kb, but you can upload files all the way up to 25mb in size. This is, however, discouraged, as such a file would take a very long time for anyone to download, especially on mobile connections.