createWriteStream to memory buffer and push to disk later - node.js

Question

I'm really struggling with connect-busboy and stream writing. The problem is I want to do some file validation but I can't do that without processing the data first and it seems like I have to write this to a file while I process it.
When the data comes in and I come across errors like file size or file types restrictions, I reject the request but a portion of the file is already written to disk. I really want to omit writing until the very end or perhaps stage it into a memory buffer.
This code will reject on invalid file types and size limits, but it always first creates the write stream and pipes the file into it. What I really want to do it move the "file.pipe(fstream);" into the file.on('end',... callback but that of course does not work because it has to be written as it comes in. So it seems like I need to first write it to a staging buffer, then on end flush it to a real file. Alternatively I could write to a temporary file, and on error delete that file, but that seems far more hacky and fragile. What is the best practice for this seemingly common problem?
saveFiles: function(request, options) {
return new Promise(function(resolve, reject) {
var fstream;
var totalData = 0;
request.pipe(request.busboy);
var fileIds = [];
var assets = {'assets': {}};
request.busboy.on('file', function(fieldname, file, originalName, encoding, contype) {
file.on('limit', function() {
var limitBytes = request.busboy.opts.limits.fileSize;
var limitMB = Math.round((limitBytes / 1048576) * 100) / 100;
return reject({errorMsg: 'Files exceeded the size limit of ' + limitMB + 'MB'});
});
file.on('data', function(data) {
if (totalData === 0) {
var type = fileType(data);
if(type) {
if (request.busboy.opts.limits.validFileTypes.indexOf(type.ext) === -1) {
return reject({errorMsg: 'Invalid file type "' + type.ext + '"'});
}
}
else {
return reject({errorMsg: 'Invalid file type'});
}
}
totalData += data.length;
});
var fileId = uuid.v1(); // used for file name on server
var parts = originalName.split('.');
var extension = parts[parts.length - 1].toLowerCase();
var fileName = fileId + '.' + extension;
fstream = fs.createWriteStream(__assets + fileName);
file.pipe(fstream);
file.on('end', function() {
assets['assets'][fileId] = {
fileName: fileId + '.' + extension,
originalName: originalName
};
});
});
request.busboy.on('finish', function(){
resolve(assets);
});
});
}