Set mtime of downloaded media

[gsnedders]

see, e.g.:

% curl -I 'https://pbs.twimg.com/media/C8Q2mOpXUAAkWz-.jpg:orig'
HTTP/2 200 
access-control-allow-origin: *
access-control-expose-headers: Content-Length
cache-control: max-age=604800, must-revalidate
content-type: image/jpeg
date: Sun, 11 Dec 2022 22:35:30 GMT
last-modified: Fri, 31 Mar 2017 17:24:25 GMT
perf: 7626143928
server: ECS (lhb/63BC)
server-timing: x-cache;desc= MISS,x-tw-cdn;desc=VZ,edge;dur=229
strict-transport-security: max-age=631138519
surrogate-key: media media/bucket/4 media/847862660247015424
timing-allow-origin: https://twitter.com, https://mobile.twitter.com
x-cache: MISS
x-connection-hash: 7a150a22da2f7c9980c2ec37b6019da5a8932b5d3760fee0286d6f7664082fa2
x-content-type-options: nosniff
x-response-time: 215
x-transaction-id: 9e85c2cb4364064a
x-tw-cdn: VZ
x-tw-cdn: VZ
content-length: 54300

It feels like it would be sensible to set mtime to last-modified, which would provide some means to get a meaningful sort order within the asset directory.