Not something you should use normally. Probably not even worth remembering, but you already did

I remembered them because I have a bit of legacy code that produces them from an HTTP file upload...

$file{$fileid, 'filename'};
$file{$fileid, 'content'};
[download]

I don't use it for new stuff but there is quite a bit of existing code that still uses so I have to deal with it on occasion. I am guessing that's what I read that shouldn't be used now rather than multidimensional arrays.