Uploading multiple images to Google Cloud Storage using Node.js
Looks good. A few nits:


You might as well use const where you can, and let when that doesn't work. You can avoid var completely.
Instead of var filePath = 'ImagesToUpload/'-- and then modifying filePath with +=, just set filePath to the correct path immediately, ie. const filePath = 'ImagesToUpload' + file. This is just simpler and easier to read.
A subtler point: The logs are providing you your only reference to the file mapping onto the server. I'm concerned that these async functions won't necessarily run in order. Consider a huge file followed by a small file. Would the loop of files allow bucket.upload calls to be running at the same time? I would assume so. The small one would complete before the large one. If this happens, it's possible that your log console.log(file) will happen out of sequence with the console.log(publicUrl). This can be solved by moving the first console log into the loop right before the publicUrl log. 
Unused arguments at the end of the list can be omitted safely in JS, so function( file, index ) can be simply function( file), or even file =&gt; if you use the new ES syntax (which you are)