[O] How to make Org-mode support file completion for file+sys: prefix?

[stardiviner]

I found Org-mode support file completion in `file:` prefix. But not `file+sys:`,
Why?

- [ ] If it is not enabled by default. Then how to enable it?
- [ ] If Org-mode does not support it, then how can I enable it?

Why do I need it?
When I add a link to a PDF file, I need to use filename completion, so I have to
use `file:` completion, then use =[C-c C-l]= to edit the link again to add 
`+sys`
for `file:`, this is painful.