Update tempFileHandler.js

Use path module to deal with paths to temp directory and temp files.

lib/tempFileHandler.js

@@ -1,12 +1,13 @@
 const fs = require('fs');
+const path = require('path');
 
 module.exports.tempFileHandler = function(options, fieldname, filename) {
-  const dir = options.tempFileDir || process.cwd() + '/tmp/';
+  const dir = path.normalize(options.tempFileDir || process.cwd() + '/tmp/');
 
   if (!fs.existsSync(dir)) {
     fs.mkdirSync(dir);
   }
-  let tempFilePath = dir + 'tmp' + Date.now();
+  let tempFilePath = path.join(dir, 'tmp' + Date.now());
   let writeStream = fs.createWriteStream(tempFilePath);
   let fileSize = 0; // eslint-disable-line