Thanks for following up. I just gave that sample a try myself, and it worked fine for me. I was able to upload multiple files of various sizes correctly.
Can you let me know more about where/how you're running this? For the example, did you follow the instructions here for running it?
And for your own code, what is req.files.file exactly? Can you step in with a debugger to check the value? (You may also want check the actual 15 bytes of content that is uploaded to see what it is actually.)
Can you also double check if the original local file you're uploading is itself accidentally only 15 bytes?