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Also the Mac Firefox combination might be an issue. There are some topics found through google on this. In addition, the flash player 10 has some new restrictions on the file upload. File upload can only be started on an event started by the user.
yes, i have faced the same problem,
the FileReference.upload() function will be worked only for flash 10.0.0, so you could update your flash,
in my case i is working fine, try it.
Thank you for all the help and confirmations.
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Since this is a flash player issue with FF on Mac, and I can't ask user to downgrade their player if needed, I probably will go outside of Flex and perform the upload.