HI All - I've been having same problem (loading photos up to Ebay)
I've read many of the threads concerning this, and have worked through suggestions.
IT IS NOT A MOJAVE issue as such...following advice above I reloaded Mojave 10.14.6... upload worked fine (using the installed Safari 12.1.2)
IT IS NOT A SECURITY UPDATE issue...following reload of Mojave and Safari 12.1.2, I then in sequence loaded the 2 security updates 2020-005 and 2020-006....again no problem upload worked.
IT IS NOT A MOJAVE SUPPLEMENT ISSUE...... I then added the macOS Mojave Supplemental Update....this introduced Safari 14.0....again no problem uploads continued to work.
IT IS....IT IS....a SAFARI 14.0.1 ISSUE....so finally I added the Safari 14.10.1 update... lo and behold uploads stopped working.
So I have no solution, but my workaround keeps me on Safari 14.0 and therefore functional, 14.0.1 clearly doesn't work with Mojave, and maybe not with Catalina (according to some posts).
Hope this helps... I've logged with Apple Support.