Trace: screenshots are PNG files saved with a .jpeg name #505

Part of #494 (found while measuring trace size).

Problem

Per-action screenshots in a trace zip are named resources/<sha1>.jpeg, but the files are PNG: on the WDIO cucumber example's trace, every one checked is PNG image data, 1600 x 757. A viewer that derives the content type from the extension serves a PNG as image/jpeg.

Switching the format is not the fix: on those 12 screenshots PNG is the smallest (724 KB) against JPEG at quality 60 (833 KB) and 80 (1.04 MB), since flat UI pages compress well as PNG.

Proposal

Name each resource by its actual format.