markdown: Fix jpeg extension in `IMAGE_EXTENSIONS`.
markdown: Fix jpeg extension in `IMAGE_EXTENSIONS`.

zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip