when there are torrent include so many separate files,, it is a very good idea to know availability for those items, this option is very helpful when downloading and old torrent or almost nearly dead torrent with no 100% available for all items

Thanks

I think this is very good idea.

Yes, I think it is good idea too, then you know which item you can download and and which item not...

Availability is for the transfer as a whole. For this we'd need a new column under Files...
So where is the cutoff? Since you know for any given file there are at least two pieces not valid for that file. They are called cross-file pieces, and have the pre- and post- parts of the file.