Autocompletion for non .md files/file paths and "gx" support

### 🚀 The feature, motivation and pitch

I would like to create links like this with autocompletion enabled. Autocompletion could be shown in the parantheses portion to avoid crowding the existing autocomplete.
`[video link](/home/user/video123.mp4)`

And to open the file when gx is used on the link.

### Additional context
_No response_

### Alternatives

_No response_

Start by locating the existing Markdown-link autocomplete and the handler used for gx. Confirm how file paths are currently filtered, then define completion for non-.md paths and opening those links with gx; done means both behaviors work without crowding the existing autocomplete.

Written by the indexing model from the issue text.