I don't think Abr's suggestion will work. (The AS argument is used when pasting, not when copying, and it specifies the filename not the format. That command will probably just do the same as Ctrl-C would normally, which isn't what you want.)
While Opus has the built-in ability to paste text (and images) from the clipboard into new files, I don't think it has the ability to do the reverse (read a file's contents into the clipboard). However, you can easily do this using the little SetClip.exe tool that I wrote:
pretentiousname.com/miscsoft ... ml#SetClip
That page on SetClip.exe includes an example Opus button (see here for instructions on pasting the code on to your toolbar) which should do exactly what you want, provided you have copied SetClip.exe to the recommended location.