I'm opened /filelist just now from RPIEasy hosted by a Raspberry Pi Zero W.
(As browser i used a Firefox v63.0 on a desktop Ubuntu Linux OS.)
Clicked to Upload, browsed a "TLS.pem" file (~1.5kb size) from my desktop, clicked Upload, and TLS.pem appeared at the RPIEasy files/ directory with no problem.
To investigate it, please feel free to open a new issue at the github.
1. Update your RPIEasy from Github
2. Set Tools->Advanced->Web Log level to "Debug"
3. Retry uploading the problematic file and post the logs. (Also describe details about the OS, browser, complete path name of the file on your system, and attach the problematic file if possible)