Bug: Uploading a shorter file after uploading a bigger file doesn't truncate it

[MrYakobo]

The bug: By uploading a shorter file after uploading a bigger file, the original file bytes are still there.
Expected behaviour: The new file should be truncated first.

To reproduce

Caddyfile

{
    order upload before file_server
}

http://localhost {
    root * ./uploads

    upload method POST {
        dest_dir ./uploads
        file_field_name f
        max_filesize 512M
    }
}

echo '{"yo": "this is my original file, that is pretty long"}' > my_file.json
curl -F f=@my_file.json http://localhost
echo '{"yo": "this is short"}' > my_file.json
curl -F f=@my_file.json http://localhost
cat ./uploads/my_file.json
{"yo": "this is short"}inal file, that is pretty long"}

Obviously, I expected the output {"yo": "this is short"} instead of a broken JSON file.

Possible errendous line

tempFile, tmpf_err := os.OpenFile(u.DestDir+"/"+handler.Filename, os.O_RDWR|os.O_CREATE, 0755)

Adding os.O_TRUNC to the options may fix the issue. I've not tested it.