Steps to reproduce:
1. A file named anyfile.anyext in any size exists on local disk.
2. Use save method of FileReference to save some data with the same file name in the same location of the anyfile.anyext.
3. Alert 'replace file?' pops, click on ok.
Actual Results:
The saving of anyfile.anyext failed, The file size is zero.
Expected Results:
The anyfile.anyext should be replaced with new data.
Workaround (if any):
When debug, traced an IOError event. I have blogged this bug in my article
http://www.tan66.com/flash-player-10-filereference-save-cant-overwrite-local-file-bug/