Proxmox content=import rejects Ubuntu cloud '*.img' with 'invalid filename
or wrong extension' though the format is qcow2. Rename non-import-extension
files to .qcow2 before download-url; import-from auto-detects the format.
Applied to both download and template workers (download filename + import volid).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>