Motorola Nexus 6 won’t show its filesystem

I’ve had a Motorola Nexus 6 for some time now, and I tend to be a bit static with it — I’d uploaded the majority of my music library at the time and then didn’t change much for a long time.

Now I recently purchased a number of new songs and wanted to upload them to my phone, but when I plug it into a computer (any computer I’ve tried so far, two linux systems and one windows 10) they recognize the Nexus 6, but when I try to open it in a file manager they only show an empty folder. Rebooting either the computer or the phone had no effect.

When I attempt the same thing on my Samsung Galaxy tablet, the tablet prompts me to enable sharing and after I confirm that I can browse the filesystem on my tablet, but the Nexus 6 phone never shows me that dialog.

It’s entirely possible that I accidentally enabled a security feature and forgot about it; can anyone offer suggestions where I should start looking?