As far as I know, there isn't a way to do this, at least on our side. There may be a way to tell Outlook to convert email sent via MAPI into HTML, but I'm not an expert there so I don't know for sure.
(By the way, the "/O" should not be part of the command. Just Copy SENDMAIL on its own is the correct form, or Copy SENDMAIL="joe.blogs@example.com" to specify a recipient via the command.)
...it will create an HTML-formatted email. I tested this and it worked fine on my system (Windows 10 with Outlook 2010).
The /a command-line argument is used for attaching a file. It appears to be limited to just one file. I tested this every way I could think of, and it's either one file or an error.
I made the attached button for you to import, if you wish. The button has the following code:
@nodeselect
@firstfileonly
OUTLOOK /c ipm.note /a {filepath}
If this doesn't work for you, it's probably because your system is not automatically finding the path to Outlook.exe. You'll need to replace "OUTLOOK" at the beginning of the third line with the full path to your Outlook.exe. For example, mine is: