ANSWER - method to downgrade from Safari 14.0.1/Mojave (multiple problems eg loading files)

Here's how to uninstall Safari 14.0.1 and revert to Safari 14.0, when in Mojave - having problems uploading files in Ebay/GMail/Yahoo/Craigslist etc etc

There are many many threads complaining of problems, I have posted this answer in some of them, but people are not finding it.

This is my solution it works for me, and it seems for others as well - there might be a better solution - but Apple need to fix 14.0.1 in Mojave.

ANSWER - reinstall mojave, install the macOS supplementary update and the security updates - DO NOT upgrade to 14.0.1, make sure to UNCHECK the Sarai 14.0.1 update every time.

DETAIL-

1. Back up Mac with Time Machine etc.
2. Download Apple MacOS Mojave installer from here macOS Mojave 10.14
3. This will give you mojave 10.14.6 and Safari 12.1.2
4. Goto System Preferences, Software Update.
5. Underneath the Catalina update notification, there will be a sentence "Other updates are available", click on the more info link, and then UNCHECK the Safari 14.0.1 - you can install/update the rest.
6. You will be left with an up to date Mojave 10.14.6, Safari 14.0 and all the security updates (you might need to run update again to get second security update - REMEMBER to UNCHECK Safari 14.0.1 update)

Its a slow process on my old iMac late 2012, but it is straightforward.

Just remember to keep unchecking the Safari 14.0.1 install, in the software update pane (in my case underneath the catalina update blurb) after the mojave reinstall.

Safari 14.0 comes bundled in the macOS supplementary update - and that's where I've left it for now.

The 2 security updates are fine to install.

good luck