To find out what gets called by menu items in an edit window, you'll need to look in <editor_install_path>/lib/editmenu.cf. You'll find a set of lines that looks like this:
Menu "File.Publish" \
active "main::activeFileComposeMenu() == 1"
{
Somewhere between that opening brace and the corresponding closing brace should be what you're looking for. Sure enough, a few lines down is this:
"HTML &File..." \
cmd "ComposeHTMLFile" active "main::activeComposeHtmlFile() == 1"
So this tells you the command or alias associated with your menu item is ComposeHTMLFile. You can also use a good text editor to search through the <editor_install_path>/packages directory tree looking for a particular string in ACL files. Doing that for the string "alias ComposeHTMLFile" shows that the commands.acl file defines the alias as pointing to the function compose_htmlfile(). Sometimes searching through the same directory for that function name will show how it's defined in ACL. In this case, compose_htmlfile() isn't defined where we have access to it, but if you look it up in Help Center you'll find the description for it.