When you click an attachment in the attachment filed on the header toolbar above a message and goto "Open with", you're going to see all the applications you have installed that have registered themselves in
Windows as supporting pdf files. As in, that list is auto-populated based on the Windows registry.
The only thing you can do is mess with:
[Mail Attachment Toolbar Popup] Item, MI_IDM_Open = Open Submenu, M_OPEN_WITH, Open in menu Item, M_SAVE_TO_DOWNLOAD_FOLDER = Save attachment to download folder Item, MI_IDM_SaveAs = Savein standard_menu.ini.
You can remove the submenu line to get rid of "open with" completely.
Then, if you need an entry to open with a specific program, you might be able to add it. It might be something like:
Item, "Open with Chrome" = Execute program, "C:\Program Files\Google\Chrome\Application\chrome.exe", "%something"
where "something" is one of the variables mentioned at https://web.archive.org/web/20080216223715/http://operawiki.info/IniFileVariables. However, I don't know if any of those variables will give you the path to the attachment in your temp directory. You might only be able to get the URL of the attachment that looks like attachment://stuff that is of no use to another program.