upload: Remove incorrect animated GIF asserts.
upload: Remove incorrect animated GIF asserts.

GIF files can be `.GIF`, and also we determine the file format by
inspecting the image data, so there's no reason to have this
assertion.

(The code for serving still images does not rely on the file being a
GIF.)

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