FIX: Use random file name for temporary uploads (PR #14250)

Other locale characters in file names (e.g. é, ä) as well as special characters can cause issues on S3, notably the S3 copy object operation does not support these special characters. Instead of storing the original file name in the key, which is unnecessary, we now generate a random file name with the original extension for the temporary file and use that for all external upload stub operations.