How to retain original file name when writing to s3?

You could use Amazon S3 executor and rename the file after it’s been written.

Also the question depends on what format you’re using when reading files from SFTP. With Whole Files it’s easy, but if it’s something else than Whole File, you may not even have the file’s boundaries preserved, ie the file may be split (due to file size limits you configured in S3 destination) or merged with other source files.