Yes, Robert is correct. You need to change your Post-processor command to something like:
/home/hts/tvheadend_convert_DRAFT.sh "%f" "%b"
and then in your script replace "%f" with "$1" and "%b" with "$2"
Again, look at the script I pointed to above as an example - at the top of his script it shows what he used as the Post-processor command.
Also, you may want to add $1 or $2 to the email in the case you have multiple transcodings...
Which leads me to another thought. If you ever have multiple simultaneous transcodings, you should change the name of the logfile to be unique (i.e. maybe at $2.)