POST upload example:

post_upload.htm
===============
<html>
<body>
<form action=/cgi-bin/post_upload.cgi method=post enctype=multipart/form-data>
File to upload: <input type=file name=file1> <input type=submit>
</form>


post_upload.cgi
===============
#!/bin/sh

# POST upload format:
# -----------------------------29995809218093749221856446032^M
# Content-Disposition: form-data; name="file1"; filename="..."^M
# Content-Type: application/octet-stream^M
# ^M    <--------- headers end with empty line
# file contents
# file contents
# file contents
# ^M    <--------- extra empty line
# -----------------------------29995809218093749221856446032--^M

file=/tmp/$$-$RANDOM

CR=`printf '\r'`

# CGI output must start with at least empty line (or headers)
printf '\r\n'

IFS="$CR"
read -r delim_line
IFS=""

while read -r line; do
    test x"$line" = x"" && break
    test x"$line" = x"$CR" && break
done

cat >"$file"

# We need to delete the tail of "\r\ndelim_line--\r\n"
tail_len=$((${#delim_line} + 6))

# Get and check file size
filesize=`stat -c"%s" "$file"`
test "$filesize" -lt "$tail_len" && exit 1

# Check that tail is correct
dd if="$file" skip=$((filesize - tail_len)) bs=1 count=1000 >"$file.tail" 2>/dev/null
printf "\r\n%s--\r\n" "$delim_line" >"$file.tail.expected"
if ! diff -q "$file.tail" "$file.tail.expected" >/dev/null; then
    printf "<html>\n<body>\nMalformed file upload"
    exit 1
fi
rm "$file.tail"
rm "$file.tail.expected"

# Truncate the file
dd of="$file" seek=$((filesize - tail_len)) bs=1 count=0 >/dev/null 2>/dev/null

printf "<html>\n<body>\nFile upload has been accepted"