I like the multipart aspect but the key signing and putting your keys in plaintext seems unnecessary.

Here is an alternative: make a bucket with a policy where it will accept anonymous uploads (but not reads) and automatic object deletion after say 24 hours. You can then on the backend just copy the object to it's final destination. The simplicity means it becomes very versatile.

There is of course a risk that somebody will maliciously dump a huge file on you or many but it won't cost you anything because the files get automatically deleted. Furthermore, none of the files are publicly readable.