WebFileUploader.RemoveAllFiles
From Xojo Documentation
Method
RemoveAllRows()
New in 2020r1
Supported for all project types and targets.
New in 2020r1
Supported for all project types and targets.
Removes all files from the upload queue.
Notes
If an upload is already in progress, any files that had been added before the upload began will still be uploaded even if 'RemoveAllRows is called. This method is useful in cases where after an initial upload has begun and the user wants to prepare for another upload.