|what you type||what it means|
|[[file filename | custom-text]]||produces a link to a file attached to this page.
custom-text changes the name of a link (custom-text will be displayed instead of the file name).
|[[file /another-page/filename | custom-text]]||produces a link to a file attached to another-page|
The destination file must be first attached to the page — by clicking "files" and "upload file" from the options at the bottom of any page. Do not use a leading slash on filenames, unless you really mean that they are attached to the default (start) page.