test_upload: Remove GIF file extension test.
test_upload: Remove GIF file extension test.

This change should have been in the previous commit.

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