If you are in Full Trust then you are seeing a progress bar when you upload? If so it means you are in full trust and using NeatUpload so the max upload size is based on this section but by default it already alllows files larger that 3.2mb
<neatUpload xmlns="http://www.brettle.com/neatupload/config/2008"
useHttpModule="true"
maxNormalRequestLength="4096"
maxRequestLength="2097151"
multiRequestUploadHandlerUrl="~/NeatUpload/MultiRequestUploadHandler.ashx"
maxUploadRate="300"
>
</neatUpload>
The Thread was being aborted message make me think the request times out or the application is being recycled before the file finishes uploading and this could happen even if large files are allowed. I would check with your host to find out if they are enforcing a short request timeout. Or if they may have the application pool configured to recycle if memory use reaches a certain level so if this happens during file upload it will not finish the upload because when the app is recycled it kills the executing thread.
Hope it helps,
Joe