Problem statement

During downloads, we put plugins as-is on the disk with their name and extension.
Issue is, download URLs end with .hpi. This is inconsistent with what Jenkins does renaming to .jpi since ~1.5xx.
I saw that a lot in JENKINS-54358, not sure where this comes from, but fixing it would at least avoid this situation in the future.

Expected

We should actively rename to .jpi after download.