function accumulate(helper, updateCallback) {
    return new external.Promise(function (resolve, reject){
        var dataArray = [];
        var chunkType = helper._internalType,
            resultType = helper._outputType,
            mimeType = helper._mimeType;
        helper
        .on('data', function (data, meta) {
            dataArray.push(data);
            if(updateCallback) {
                updateCallback(meta);
            }
        })
        .on('error', function(err) {
            dataArray = [];
            reject(err);
        })
        .on('end', function (){
            try {
                var result = transformZipOutput(resultType, concat(chunkType, dataArray), mimeType);
                resolve(result);
            } catch (e) {
                reject(e);
            }
            dataArray = [];
        })
        .resume();
    });
}
Listen a StreamHelper, accumulate its content and concatenate it into a
complete block.
@param {StreamHelper} helper the helper to use.
@param {Function} updateCallback a callback called on each update. Called
with one arg :
- the metadata linked to the update received.
@return Promise the promise for the accumulation.