There are "@" issue on multipart POST requests.

Solution for PHP 5.5 or later:
- Enable CURLOPT_SAFE_UPLOAD.
- Use CURLFile instead of "@".

Solution for PHP 5.4 or earlier:
- Build up multipart content body by youself.
- Change "Content-Type" header by yourself.

The following snippet will help you :D

<?php

/**
 * For safe multipart POST request for PHP5.3 ~ PHP 5.4.
 * 
 * @param resource $ch cURL resource
 * @param array $assoc "name => value"
 * @param array $files "name => path"
 * @return bool
 */
function curl_custom_postfields($ch, array $assoc = array(), array $files = array()) {
    
    // invalid characters for "name" and "filename"
    static $disallow = array("\0", "\"", "\r", "\n");
    
    // build normal parameters
    foreach ($assoc as $k => $v) {
        $k = str_replace($disallow, "_", $k);
        $body[] = implode("\r\n", array(
            "Content-Disposition: form-data; name=\"{$k}\"",
            "",
            filter_var($v), 
        ));
    }
    
    // build file parameters
    foreach ($files as $k => $v) {
        switch (true) {
            case false === $v = realpath(filter_var($v)):
            case !is_file($v):
            case !is_readable($v):
                continue; // or return false, throw new InvalidArgumentException
        }
        $data = file_get_contents($v);
        $v = call_user_func("end", explode(DIRECTORY_SEPARATOR, $v));
        $k = str_replace($disallow, "_", $k);
        $v = str_replace($disallow, "_", $v);
        $body[] = implode("\r\n", array(
            "Content-Disposition: form-data; name=\"{$k}\"; filename=\"{$v}\"",
            "Content-Type: application/octet-stream",
            "",
            $data, 
        ));
    }
    
    // generate safe boundary 
    do {
        $boundary = "---------------------" . md5(mt_rand() . microtime());
    } while (preg_grep("/{$boundary}/", $body));
    
    // add boundary for each parameters
    array_walk($body, function (&$part) use ($boundary) {
        $part = "--{$boundary}\r\n{$part}";
    });
    
    // add final boundary
    $body[] = "--{$boundary}--";
    $body[] = "";
    
    // set options
    return @curl_setopt_array($ch, array(
        CURLOPT_POST       => true,
        CURLOPT_POSTFIELDS => implode("\r\n", $body),
        CURLOPT_HTTPHEADER => array(
            "Expect: 100-continue",
            "Content-Type: multipart/form-data; boundary={$boundary}", // change Content-Type
        ),
    ));
}

?>

For PHP < 5.5:

<?php

if (!function_exists('curl_file_create')) {
    function curl_file_create($filename, $mimetype = '', $postname = '') {
        return "@$filename;filename="
            . ($postname ?: basename($filename))
            . ($mimetype ? ";type=$mimetype" : '');
    }
}

?>