Debug Information

https://github.com/zkoss/zk/blob/ec559349801fcc31e7cd5b2ccea14714fc249479/zk/src/org/zkoss/zk/au/http/AuUploader.java#L442

AuUploader and AuDropUloader use common fileupload to retrieve uploaded medias during parseRequest()

If the common-upload library encounter an issue during sfu.parseRequest(request), it may throw a FileUploadException

This exception is caught in the try catch block located in the service method

https://github.com/zkoss/zk/blob/ec559349801fcc31e7cd5b2ccea14714fc249479/zk/src/org/zkoss/zk/au/http/AuUploader.java#L167-L193

In the exception is caught in this block, there is no client alert, or logging available for FileUploadException (it doesn't trigger any of the existing exceptions)

Improvement

Add a special handling case for FileUploadException which logs to logger