Cannot create menu items linking to existing files

Description of the bug I'm in a little over my head here. But, I'll report this and we'll see if it makes sense.

As I described in the forum I tried to create a menu link directly to a PDF in the file system, but I get an error message saying "The path '...' is either invalid or you do not have access to it."

I would expect to be able to create a menu link to a file uploaded to Backdrop.

In this issue @jenlampton said the following:

the menu system checks links to make sure they are registered system paths. We need to remove that validation too.

Which lead me to open this ticket and call it a bug (which might not be accurate).

Steps To Reproduce To reproduce the behavior: 1. Create a file field on a content type. 2. Add a file to a node and save 3. copy the relative path to the file in the file directory 4. Try to create a menu item linking to that file.