Using multipart post to upload file with parameters.

upload.as

var urlRequest:URLRequest = new URLRequest();
urlRequest.url = uploadPath;
urlRequest.contentType = 'multipart/form-data; boundary=' + UploadPostHelper.getBoundary();
urlRequest.method = URLRequestMethod.POST;
urlRequest.data = UploadPostHelper.getPostData(file, parameters);
urlRequest.requestHeaders.push( new URLRequestHeader( 'Cache-Control', 'no-cache' ) );

var urlLoader:URLLoader = new URLLoader();
urlLoader.dataFormat = URLLoaderDataFormat.BINARY;
urlLoader.addEventListener(Event.COMPLETE, onComplete);
urlLoader.addEventListener(IOErrorEvent.IO_ERROR, onError);
urlLoader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, onError);
urlLoader.load(urlRequest);

package {

    import flash.events.*;
    import flash.net.*;
    import flash.utils.ByteArray;
    import flash.utils.Endian;

    /**
     * Take a fileName, byteArray, and parameters object as input and return ByteArray post data suitable for a UrlRequest as output
     *
     * @see http://marstonstudio.com/?p=36
     * @see http://www.w3.org/TR/html4/interact/forms.html
     * @see http://www.jooce.com/blog/?p=143
     * @see http://www.jooce.com/blog/wp%2Dcontent/uploads/2007/06/uploadFile.txt
     * @see http://blog.je2050.de/2006/05/01/save-bytearray-to-file-with-php/
     *
     * @author Jonathan Marston
     * @version 2007.08.19
     *
     * This work is licensed under a Creative Commons Attribution NonCommercial ShareAlike 3.0 License.
     * @see http://creativecommons.org/licenses/by-nc-sa/3.0/
     *
     */
    public class UploadPostHelper {

        /**
         * Boundary used to break up different parts of the http POST body
         */
        private static var _boundary:String = "";

        /**
         * Get the boundary for the post.
         * Must be passed as part of the contentType of the UrlRequest
         */
        public static function getBoundary():String {

            if(_boundary.length == 0) {
                for (var i:int = 0; i < 0x20; i++ ) {
                    _boundary += String.fromCharCode( int( 97 + Math.random() * 25 ) );
                }
            }

            return _boundary;
        }

        /**
         * Create post data to send in a UrlRequest
         */
        public static function getPostData(fileName:String, byteArray:ByteArray, parameters:Object = null):ByteArray {

            var i: int;
            var bytes:String;

            var postData:ByteArray = new ByteArray();
            postData.endian = Endian.BIG_ENDIAN;

            //add Filename to parameters
            if(parameters == null) {
                parameters = new Object();
            }
            parameters.Filename = fileName;

            //add parameters to postData
            for(var name:String in parameters) {
                postData = BOUNDARY(postData);
                postData = LINEBREAK(postData);
                bytes = 'Content-Disposition: form-data; name="' + name + '"';
                for ( i = 0; i < bytes.length; i++ ) {
                    postData.writeByte( bytes.charCodeAt(i) );
                }
                postData = LINEBREAK(postData);
                postData = LINEBREAK(postData);
                postData.writeUTFBytes(parameters[name]);
                postData = LINEBREAK(postData);
            }

            //add Filedata to postData
            postData = BOUNDARY(postData);
            postData = LINEBREAK(postData);
            bytes = 'Content-Disposition: form-data; name="Filedata"; filename="';
            for ( i = 0; i < bytes.length; i++ ) {
                postData.writeByte( bytes.charCodeAt(i) );
            }
            postData.writeUTFBytes(fileName);
            postData = QUOTATIONMARK(postData);
            postData = LINEBREAK(postData);
            bytes = 'Content-Type: application/octet-stream';
            for ( i = 0; i < bytes.length; i++ ) {
                postData.writeByte( bytes.charCodeAt(i) );
            }
            postData = LINEBREAK(postData);
            postData = LINEBREAK(postData);
            postData.writeBytes(byteArray, 0, byteArray.length);
            postData = LINEBREAK(postData);

            //add upload filed to postData
            postData = LINEBREAK(postData);
            postData = BOUNDARY(postData);
            postData = LINEBREAK(postData);
            bytes = 'Content-Disposition: form-data; name="Upload"';
            for ( i = 0; i < bytes.length; i++ ) {
                postData.writeByte( bytes.charCodeAt(i) );
            }
            postData = LINEBREAK(postData);
            postData = LINEBREAK(postData);
            bytes = 'Submit Query';
            for ( i = 0; i < bytes.length; i++ ) {
                postData.writeByte( bytes.charCodeAt(i) );
            }
            postData = LINEBREAK(postData);

            //closing boundary
            postData = BOUNDARY(postData);
            postData = DOUBLEDASH(postData);

            return postData;
        }

        /**
         * Add a boundary to the PostData with leading doubledash
         */
        private static function BOUNDARY(p:ByteArray):ByteArray {
            var l:int = UploadPostHelper.getBoundary().length;

            p = DOUBLEDASH(p);
            for (var i:int = 0; i < l; i++ ) {
                p.writeByte( _boundary.charCodeAt( i ) );
            }
            return p;
        }

        /**
         * Add one linebreak
         */
        private static function LINEBREAK(p:ByteArray):ByteArray {
            p.writeShort(0x0d0a);
            return p;
        }

        /**
         * Add quotation mark
         */
        private static function QUOTATIONMARK(p:ByteArray):ByteArray {
            p.writeByte(0x22);
            return p;
        }

        /**
         * Add Double Dash
         */
        private static function DOUBLEDASH(p:ByteArray):ByteArray {
            p.writeShort(0x2d2d);
            return p;
        }

    }
}