Websites adding "torrent downloaded from.." files

[MaxiSongs]

I was wondering how websites are able to add those "torrent downloaded from SomeWhere.com.txt" text files. I thought once a torrent was created and its hash established, you could not modify any files in it.

However, when uploading to Extratorrent, Kickass, even some older TPB torrents have a text file stating "torrent downloaded from...."

How can the website add files to torrents that have already been created?

[Of course, I'd never want to do such a thing, I was wondering how it was technically possible.]

[SubZero]

I was wondering how websites are able to add those "torrent downloaded from SomeWhere.com.txt" text files. I thought once a torrent was created and its hash established, you could not modify any files in it.

However, when uploading to Extratorrent, Kickass, even some older TPB torrents have a text file stating "torrent downloaded from...."

How can the website add files to torrents that have already been created?

[Of course, I'd never want to do such a thing, I was wondering how it was technically possible.]

I don't think the website does this. The uploader would have to add these files.

[NIK]

The websites could do this, quite easily. When you upload a file to TPB the upload script edits it--removing trackers that are blacklisted and adding the trackers TPB currently supports. The file list is merely another entry within the .torrent file and it could be edited in exactly the same way.

But

SubZero is correct. No site that I have ever heard of has ever done this. Instead, they tell uploaders to include such files and, when torrent files are uploaded, any which don't include the file are rejected and an error message thrown.