I am doing further testing and it now responding with the correct content type. I have made no changes in configuration. Maybe I was imagining it before.
curl --head https://....jpg
HTTP/2 200
accept-ranges: bytes
content-type: image/jpeg
etag: W/"...-8c921"
last-modified: Wed, 01 Nov 2017 12:20:31 GMT
server: Caddy
content-length: 575777
date: Thu, 02 Nov 2017 01:05:48 GMT
curl --head https://....jpg -H "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"
HTTP/2 200
accept-ranges: bytes
content-type: image/webp
etag: W/"...-abc8"
last-modified: Wed, 01 Nov 2017 12:20:32 GMT
server: Caddy
content-length: 43976
date: Thu, 02 Nov 2017 01:07:40 GMT