This did it
Code:
retval, source_filename = reaper.GetUserFileNameForRead("", "Choose SEQ File", "seq")
file = io.open(source_filename, "rb")
source_content = file:read("*all")
source_filename = string.gsub(source_filename, ".seq$", " seq .mid")
file = io.open(source_filename", "wb")
file:write(source_content)
file:close()