[MM-29111] Fix errcheck issues in server/channels/app/upload.go (#30678)

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -97,7 +97,6 @@ issues:
         channels/app/platform/license.go|\
         channels/app/slashcommands/command_test.go|\
         channels/app/slashcommands/helper_test.go|\
-        channels/app/upload.go|\
         channels/store/localcachelayer/channel_layer.go|\
         channels/store/localcachelayer/channel_layer_test.go|\
         channels/store/localcachelayer/emoji_layer.go|\