Open a terminal and run:

cat ~/Desktop/foo.html | pbcopy

The file is now in your clipboard.

To put it somewhere else (i.e. paste it) run:

pbpaste > ~/Documents/bar.html

Now you should have a copy of foo.html sitting in ~/Documents.

Refs:

• https://apple.stackexchange.com/questions/15318/how-to-use-terminal-to-copy-a-file-to-the-clipboard