dropzone js - upload copied file leaving it open gives 'server responded with code 0'

Question

I use dropzone to upload my files.

these files are usually downloaded from my web application, modified and later uploaded to the same web application.

1. Downloaded the file from my web appln.(to downloads folder)
2. made modifications and kept the file open and dropped it in the dropzone to upload.
3. the above works fine.
4. Copied this downloaded file(step1) to another location(to my desktop)
5. repeated step2
6. here i get 'Server responded with code 0' error.

Why the dropzone throws this error only when leaving the copied files open and dropping in.