Downloading epub file

Hello. I'm having trouble downloading some of my larger-sized epub files, around 100MB, from the Opencart portal. I can ftp the files in file manager into the download storage folder. But how do I move them to show up on the Opencart download page? Or are there adjustments I can make so that they will download properly? It seems my maximum upload from my server is set at 256M and on Opencart at 999M. There are no error logs from my failed attempts to upload, but I am getting this error message:
"Internal Server Error
Internal Server Error
{"filename":"LingoMeetsWide.epub.sZX9TD0FqVQMhQOIHoA8VVxwK2xbgliE","mask":"LingoMeetsWide.epub","success":"Your file was successfully uploaded!"}". Any help is appreciated.

I think you should submit a ticket to your web host, it could well be a server timeout.