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.
Recent comments
Yes indeed. We are exploring a few other more costly options, but as we are a low-resource start-up, we could save a lot of money by integrating Backdrop, CiviCRM and Ubercart for our membership...
UberPOS for Backdrop?
Hello Chrys.., It looks like it was for Drupal 6, with a Dev release for D7... Never seen it before, but it looks like an interesting module... Do you have an immediate need...
UberPOS for Backdrop?
I do have the apache status page set up ... I will look in to this really fast. BRB.
Status Report Page throws : Forbidden You don't have permission to access this resource.