FIX: Show gif upload size limit error straight away (#21633)

When uploading images via direct to S3 upload, we were
assuming that we could not pre-emptively check the file
size because the client may do preprocessing to reduce
the size, and UploadCreator could also further reduce the
size.

This, however, is not true of gifs, so we would have an
issue where you upload a gif > the max_image_size_kb
setting and had to wait until the upload completed for
this error to show.

Now, instead, when we direct upload gifs to S3, we check
the size straight away and present a file size error to
the user rather than making them wait. This will increase
meme efficiency by approximately 1000%.

spec/requests/uploads_controller_spec.rb

@@ -853,6 +853,16 @@ RSpec.describe UploadsController do
         )
       end
 
+      it "returns 422 when the file is an gif and it's too big, since gifs cannot be resized on client" do
+        SiteSetting.max_image_size_kb = 1024
+        post "/uploads/create-multipart.json",
+             **{ params: { file_name: "test.gif", file_size: 9_999_999, upload_type: "composer" } }
+        expect(response.status).to eq(422)
+        expect(response.body).to include(
+          I18n.t("upload.images.too_large_humanized", max_size: "1 MB"),
+        )
+      end
+
       it "returns a sensible error if the file size is 0 bytes" do
         SiteSetting.authorized_extensions = "*"
         stub_create_multipart_request