It looks like it works but it's not reliable so your reliability concerns then get externalised into your application which multiplies complexity terribly.
Fun Trivia note: when you do a multipart upload, the S3 hash of the object is not the same as it is when you do a single part upload. I had a file with the same contents but a different hash when I used Python than when it was transferred with the CLI or CloudBerry. The quick and dirty way to fix the hash is to copy the file to itself with Boto3.