Fix errcheck issues in server/channels/app/imaging/decode_test.go (#28958)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -105,7 +105,6 @@ issues:
         channels/app/file_test.go|\
         channels/app/helper_test.go|\
         channels/app/imaging/decode.go|\
-        channels/app/imaging/decode_test.go|\
         channels/app/import_functions.go|\
         channels/app/import_functions_test.go|\
         channels/app/import_utils_test.go|\