I have been searching and searching and fear there is no way to do this. All answers on this and every other site have pointed me to Java, PHP, and iFrames - none of which will work in outlook.
The goal is to have a central location to update a footer and other information without having to go to each user's PC every time we need to update it. Also, the footer has a bit of formatting to it, so HTML is best, but txt/css may work, too.
Best results came from iFrame. It did what I wanted in the browser, but it doesn't work with Outlook and many other email clients.
Second best was a Java based include function I got from w3 schools, but it doesn't work on the local machine. It works fine from web when all is copied to the site, but the same folder structure on the local PC displays an empty page.
And I wish I could use PHP, but this is not going to work with Outlook.
One option is to design the email signature or footer in HTML & CSS. There are lots of resources on how to do this properly, with support for all reveiving email clients. You can call to a resource image from css or html, so every time you want to change content you just change the image on the server. This is limited to images tough, but maybe an adapted solution?