There is no need to download/upload all of the forum files to add an extension.
Using a FTP program, go to the directory that contains your phpBB files (they may all be in the HTDOCS directory, or a sub directory. Only you can know this)
When you open he correct directory you will see something like this:-
Opening the ext directory will display any uploaded extensions you have in their authors own directory, like this
This is where you upload any extensions you want to use. ( exactly as I showed you HERE
The beauty with extensions is that they do not 'mess about with' your files, but stand alone without needing any editing.