SD Card file attributes with SDFS3 ?

I am really grateful for the SDFS3.ZIP code in the file sample page.

I am using it to read files on an sd card and then transmit them over the internet via TFTP (not a security critical application)
My problem is that the first download works fine, but then if I try to download again over the top of the first file (now stored on a remote pc) I get an access error because the downloaded file has had it's read-only attribute set by SDFS3.

I'm sure it didn't happen the first few times, but which file attribute might have been changed, and how can I set it to write non-restricted files ?

Chris