Uniquely identify files before uploading with the HTML5 file API

While making a fileuploader using HTML5 file API, we want to be sure that no duplicate files are uploaded based on actual data. 

Calculating a hash with MD5 is not an efficient method as all that happen on the client side and is time-consuming. 

There is actually no shortcut for this. 

If we need to identify duplicate files with no confusion then we have to first read the content of each file and then compare it.

Another way is to find MD5 hash for given subset of file blocks using predefined invariant window.