Hi guys,
it seems we have been able to identify the bug. It is related to the way WebShield handles network buffers. Several things act together here - the size of the file/image being uploaded (>128KB) and the size of Winsock network buffer space for the connection (<128KB), together with the fact, that Firefox does not wait for the final SSL Handshake confirmation message before sending the payload.
We are working on a fix, it should be available here for testing in one or two days, when verified it'll be distributed most probably via emergency/micro updating mechanism.
Thanks.
Lukas.