I tried evaluating find-file-name-handler as suggested and it returns nil. I suspect it is due to OneDrive (and now also Nextcloud's) virtual files, which live online until requested. Somehow opening the file does not trigger the required download.

Juanjo

Lars Ingebrigtsen <larsi@gnus.org> escribió:

Juan José García-Ripoll <juanjose.garcia.ripoll@csic.es> writes:

The file is living in OneDrive in Windows 10/11. I will try with other
files in the same file system, but it has hit me hard this week, which
I am in remote mode.

As Eli says, buffer-file-name apparently being nil in this case, and
that's very odd.

Could there be a special file name handler for OneDrive that's doing
something odd?

What does

M-: (find-file-name-handler "~/OneDrive/Mail/archive/Archives-2017.mbox" 'insert-file-contents) RET
say?