Add warnining when directly uploading a file and a file with the same name exists.

[anchovy]

I think we need a warning when uploading a fie and a file with the same name already, so the use has the option to use the existing file or rename the new file.

This way, the user knows there is an existing file and they can then review this file to see if it's correct or needs to be updated, or if they do need to upload a totally different file and change the file name to be more descriptive.

See #10145