thumbnailUrl path is invalid when file is stored outside of US East

[rnicholus]

Prompted by FineUploader/fine-uploader#1761.

I think it's best to change the logic in the Fine Uploader PHP S3 repository to always return a thumbnailUrl using the virtual hosted-style path format. I can't think of a good reason to use path-style urls, unless the bucket name is not DNS-compatible (which isn't supported outside of US East anyway).