This sounds like an issue in the Mx environment itself.
I would file a support ticket for this, since the S3 settings are something you simply can not change if you use the default Mx cloud. As far as I know, Mx does not document or dictate a max file size, so if it breaks on a 30 mb upload, that's a bug...
Haven't had the same problem but you can set the max size and the type you accept in mendix. Maybe this can stop people from uploading big files.