That is good news. Thank you.

Try to find or remember the list of pairs of drive letter and bucket locations, e.g.

G: bucket1\folder2
H: bucket1\folder17

Those pairs are arbitrary and unique to your system and storage requirements.

If you did not document them, the only way to figure them out is to notice when something does not work, then figure out the missing drive letter, and then figure out where in your AWS S3 world the needed matching files would live.

FWIW I have coded a workaround for this for my uncle for the time being.

It would be extremely helpful to know a rough release schedule. Days? Weeks? Months? to next release? Is there a road-map available?

FWIW, I use CloudBerry Explorer v5.6.1.8 on Windows 10 and it uploads with correct Content-Type.

Any estimate for "a bit later" ?
I do think it's great that you have Mac support in the works.
Previously we were using S3 Organizer add-on for Firefox on Mac to upload files. It was able to set the Content-Type automatically. This worked on Mac as long as one stayed with a very old version 18 of Firefox.
Suddenly the S3 Organizer add-on stopped working. I guess one of the security protocols finally expired on the AWS side.
That leaves us in a bind, looking for an upload tool for a non-profit personal web site that my uncle maintains.
If your release will be soon, before 15 July, we'll wait. Otherwise I will need to script something on the AWS side to adjust the content type after the files arrive.
I thought possibly the "Upload Rules" feature in the 360 product was meant to handle this.
Advice welcome.