filezilla does not have the ability to open or edit files. however, it does have the ability to use whatever your default text editor is on your computer. in this case it probably used something like windows notepad and that has corrupted the file.
get a real text editor like notepad++ or editpad lite
download it , install it and only open phpbb files with that editor.
now, the easiest way to solve this problem is to get a fresh new copy of that file from the backup on your computer or download the whole phpbb from here and use the copy in that.
replace the one on your server with the new copy.
If you wish to make this edit ( I wouldn't do it because you will just have to do it again each time you update phpbb ) you can use your new text editor to do it with and you won't have this problem.
be sure to set your new text editor as the default in filezilla for the future.
I'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
Premium phpBB 3.3 Styles
If nobody is in the forest, does a tree really fall?