How can I reset the default app for opening email attachments? Every time I try to open an attachment from emails from a specific person it goes to Stumble Upon.  I think I clicked Stumble Upon and "Always" once, and now I can't undo it.  How can I stop this?
You need to clear the "Launch by default" setting for StumbleUpon.

Open up your phone settings, then go to Applications.

Look for the StumbleUpon app, and click on it. Here is where you can do things such as uninstall/disable the app, clear the data, etc.

One of the options is to Clear defaults. This is the option you need to run. It will restore the app-selection menu for any actions where you set StumbleUpon to be the default handler.