From the email, you must tap on the file to retrieve it. Then tap on the file (still in the email message) Tap on the SHARE up arrow thing in the upper right. Scroll down and choose “Save to Files”. Click SAVE in the upper right. (Or you can choose the Downloads folder.) If you did not choose Downloads, it would be in the MAIN folder of ON MY IPAD; otherwise, it will be in DOWNLOADS.
You should be able to select that file …  then … from the Settings > Upgrade firmware> Icon with Computer/Up arrow above it.