Ok, a little more progress. I set up the site locally, using IIS 7 in classic mode.

This time, the control rendered differently again:

 onload="this.style.display="none" ; var OxO6760=["CuteWebUI_AjaxUploader_Initialize","document","{0} cannot be uploaded!\x0A\x0AFile size ({1}) is too large. The maximum file size allowed is set to: {2}.","This file extensio

.....

 [182]][OxO6760[135]][OxO6760[199]]=Ox24+OxO6760[198];} ;} ;} ;CuteWebUI_AjaxUploader_Initialize(this.id);" onerror="this.onload();

So this time, there is still a LOT of javascript inserted, but it is formatted corrected, so there are no issues.