Filelink is a list of hashes of chunks of file. As Hellnet has limited chunk size1 and filelinks have to be files too, and we can't use filelinks on filelinks, Hellnet has extent-based filelinks.
If chunk size is 256 kb, we can fit 4096 hashes in it. That's 1.07 Gb of data. If file needs more than 4095 hashes to describe it, we make another filelink chunk, fill it with the rest and append hash of that chunk to first one. Can be recursively repeated.
1 ↑ Currently 256 kbytes