I tried the following steps:
1) Select a book with some existing data files
2) Edit metadata
3) Click the insert link button in the comments editor
4) click Data file
5) A file selection dialog open to the data sub-folder displays
6) choose a file
7) click ok
A link of the form <a href="data/filename"> is created, as expected.
The only bug that I could see is clicking on this link doesnt work, because its a relative link, that will be fixed.