Hey jock, let's give keyboard shortcuts a shot! (disclaimer - this is assuming you're on a desktop/laptop and it's Windows!)
Open your text file with your literary epic.
Highlight all the text - select all with the mouse or press CTRL+A on the keyboard.
Now press CTRL+C on the keyboard - this copies the text to the 'clipboard'
Open your reply window on the forum and then press CTRL+V on the keyboard - this pastes the contents of the clipboard - and this should paste your text into the window.
Hope this works for you
Edit: Ha, beaten to it by Shamus, I hate typing on my phone!!