Are you using FF to access the site and email? Also did you confirm the file was not in correct format?

I am using Firefox when I open my gmail account. I can't upload to the site until it's on my computer. The file is in the correct format. I have viewed it in Adobe Digital Editions.

Actually the epub can also be view with Win10 Edge as well or anything else that reads epub so it not limited to just Adobe digital editions.

I got help from my computer repair shop. Here's how it works on my PC and laptop with Windows 7. I hope this helps others.

Note: wherever I've typed click, depending on your setting you will single or double click

On your attached gmail mobi or epub file click the download arrow. Choose save file > okay

Open Downloads Folder (located on the taskbar bottom of your screen by the start icon)

On the left sidebar click/open Downloads Folder

If your downloaded gmail mobi or epub file doesn’t appear to the right with the list of prior downloaded folders, hover over that list > right click.

In the popup menu click details

Your mobi or epub gmail attached file should be there.