Hashing the file and receiving a cryptographic signature

[damianjnc]

I don’t know how to do what happens at 00:40 where you get a popup and have a user uploading the file and then it get stored and cryptographic signature is receive, how to create that functionality? https://vimeo.com/239155784 1

I asked on a few foums but what they said was something around the lines of
You want to include the hash of the files contents in your signed message to the blockhain

eg, on OSX terminal

$ md5 fileToNotarize.doc
MD5 (fileToNotarize.doc) = e789c8f38a10b495614ae07665c8db79
Then include the file hash with a transaction message to the blockchain

But I still don’t know how to really do it, do you know any more comprehensive sources?