Navigate to your project's name channel (the one with a lock on the left side of the name). On the message composer, you will find a staple icon. Just click on it and upload any file type that you need. There is no limitation at all. If you are on a computer, you can drag and drop a file and it will get attached to a message - send the message and that's it.
Once you have uploaded the file, send a message by mentioning Vasilis and inform him that you have uploaded this document.
Ex: @Vasilis I have uploaded this document "Document's name"