Open .xml attachments in "default program" instead of Internet Explorer

Solution 1:

After diffing my entire registry before and after the change, I found that setting Notepad++ to the default set "Progid"="Applications\\notepad++.exe" in [HKEY_USERS\S-1-5-21-73586283-1229272821-725345543-2231\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xml\UserChoice]. With UAD Reader as the default, "Progid" in the same location was set to "xmlfile".

To fix this, I set "Progid"="Applications\\alamode.uadreader.exe". This completely fixed my problem and now XML files open in UAD Reader even as attachments in Outlook.

NOTE: If you are manually editing that entry in the registry, you will need to give yourself write permission to it.

I don't know why changing this in Default Programs or setting it to Always open with... doesn't set this entry, but at least it's something that can be deployed via Group Policy.