I have the same problem. The server-log shows the folowing:
01/24/2011 03:59:10 PM HTTP JVM: DOJOX.Form.FileUpload To Amazon S3 : Started
01/24/2011 03:59:10 PM HTTP JVM: DOJOX.Form.FileUpload To Amazon S3 : Load com.amazonaws package
01/24/2011 03:59:10 PM HTTP JVM: DOJOX.Form.FileUpload To Amazon S3 : get Access To The Uploaded File
01/24/2011 03:59:10 PM HTTP JVM: DOJOX.Form.FileUpload To Amazon S3 : Upload Key :
01/24/2011 03:59:10 PM HTTP JVM: DOJOX.Form.FileUpload To Amazon S3 : Upload Days : 14
01/24/2011 03:59:10 PM HTTP JVM: error: com.ibm.jscript.InterpretException: Script interpreter error, line=23, col=89: 'fileData' is null