How to save mail attachments in Files App.

1) This depends on the app you're using, if it supports Files. But the normal procedure would be to tap the "Action" icon (the rectangle with an up arrow at the top). Tap that and then select what you want to do with the attachment ("Save to Files" is what you're looking for, and is on the bottom row of supported Actions).

2) You can add new folders from the Files app itself, but not as part of saving a document to Files. You'd have to open the Files app first, create the folder, then go back to the app you're using and send the item to the created folder.

3) When you tap the "Action" icon, scroll all the way to the right of the bottom row of actions and tap "More". On the Activities panel, you'll see available options, which may just need to be enabled. To my knowledge, you can't add to this unless you add a new app that would add a new Activity.

Not sure what you mean about Excel, but from the iOS Outlook app I can save an Excel document to Files. If you're using Outlook, be sure that you're tapping the "Action" icon mentioned above, not Outlook's "Save" icon (an arrow pointing into a cloud ... that does not support Files, but does support Dropbox, Drive, etc.)