I don't know how facebook assigns rights to overwrite files. This forum is not the right place to ask a question like that.

I would have expected that the person who first loads the file could control who had access to read/write the same file. If someone else has edited the file then they may have changed your write privileges either accidentally or on purpose.

Perhaps, once the updated file is saved to your hard drive, you can write it back over the top of the same (older) file on facebook.

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Andrew Lockton
Chrysalis Design, Melbourne Australia