Yes, as Peer said .. you need to add functionality to the erlang module. I sent a fix for an FTP module that does just that:
https://gist.github.com/700908
Basically, I found the recv_chunk_start / 3 API in the FTP module. It accepts the optional “Pos” parameter to resume FTP upload from this position.
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1) FTP- RETR ( -- )
2) erlang-R13B04.. R14B.