I made some progress on this. Reading through the code, it appeared that the minio-go package was checking for an endpoint located at "s3.amazonaws.com". Looking at amazon's endpoint documentation, I found that that had to be in region US Standard.
I sync'd my s3 bucket into a new bucket located in region US Standard and pointed mattermost to the new bucket. It's not connecting properly, but none of the files are visible (they show up as 
🏼).
It looks like there is a new database structure to store files, in a "fileinfo" table and that older format files should automatically convert, but it doesn't seem to be happening. I don't see any notes in the changelog. Does anyone have any experience with this problem?
Edit: newly added files appear correctly and add an entry into the fileinfo database