Comment 2 for bug 1616909

APT is using dpkg's --recursive option with a temporary directory since recently if it has to touch >5 packages to avoid producing too long commandlines for the kernel (yes, that is a thing… although unlikely it does happen in big upgrades). Seems like this interface in dpkg does support only *.deb files. Debian had decided to stick with .deb for their automatic debug packages to avoid changing all tools dealing with deb files to also deal with ddeb (or whatever).

Anyway, as that used to work with apt and is a valid althrough uncommon thing to have packages without .deb as extension I have just commited a patch upstream: https://anonscm.debian.org/cgit/apt/apt.git/commit/?id=4f242a2

As a workaround you can set the option "dpkg::install::recursive" to false and "dpkg::install::recursive::force" to true OR set "dpkg::install::recursive::minimum" to e.g. 100 (assuming you can accept not installing a 100 ddebs at once for the time being). Both workarounds effect all dpkg calls through, so please only use if needed until the patch reaches Ubuntu – don't set it "forever" or you will eventually run into the problems this is designed to void in the first place…