I found https://devcentral.f5.com/questions/possible-to-upload-overwrite-ifiles-using-icontrol
I had to do a couple things to make the upload work though. Here's my revised version:
!/bin/bash
filename=blah.txt
filesize=`wc -c < $filename`
echo $filesize
newfilesize=`expr $filesize - 1`
echo $newfilesize
filesize=$newfilesize
curl -k -u admin:admin \
--trace trace.txt \
--silent \
--request POST \
--header "Content-Type: text/plain" \
--header "Content-Range: 0-$filesize/$filesize" \
--data-binary "@$filename" \
$filename | python -m json.tool