Recently noticed a problem with Safari 14.0.1 in Mojave (10.14.6). I help admin a site running forum software. One frequently used functions is to attach an image to a post. Trying to do that, I discovered the (probably javascript function) “Choose File” button would not bring up the normal system/Finder file Navigation dialog.
Spent hours uninstalling/disabling/settings checking to no avail. Checked this app/OS combo on two other Mac’s (a new mini and a 2yo MBP). They all had the same problem! At least I now quit blaming myself for creating the problem! 
I finally went to the Apple Support forums and found that over 175 people were reporting the same problem with this same configuration. My cynical self sees this as another “request” to upgrade to Catalina or Big Sur. 
My logical self thinks it’s just poor quality control. I added my bug report to Apple’s “Feedback” list, but I doubt many man-hours will be spent on a problem in such an “old” OS. 