Attached files
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.