We're not aware of any mechanism capable of chmodding files within UBB.threads; if you take a tool such as wingrep and search "chmod" (which is both the command line and PHP command to change permissions) you'll see that no attachment script even makes note of it.

For the time being I'd setup a crontask (hourly?) to chmod your attachments folder 777.
Code
0 * * * * chmod 777 /path/to/public_html/forums/attachments

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