HTML5 uploader supports several ways to generate thumbnails. In addition to a classic mode when it just proportionally resizes images and fit them into specified boundaries, it also can automatically centrize and crop a thumbnail. As a result, you receive thumbnails with the same width and height, no matter what was an original aspect ratio. This approach is used to display thumbnails on mobile devices, e.g. iPhone's Camera Roll.
To create such thumbnails, just set thumbnailFitMode of an appropriate converter to crop and set thumbnailWidth and thumbnailHeight to
the desired values.
Apps\SquareThumbnails folder of the Upload Suite 8 SDK.