Relax behavior of "Download first and last piece first"

Now it applies the setting to all files in torrent, no matter whether
the file is previewable or not.

Torrent client shouldn't be smart by deciding which file this
option will be applied to, i.e. it should just follow the user's decision.

src/base/bittorrent/torrenthandle.cpp

@@ -781,10 +781,6 @@ bool TorrentHandle::hasFirstLastPiecePriority() const
         if (filePriorities[i] <= 0)
             continue;
 
-        const QString ext = Utils::Fs::fileExtension(filePath(i));
-        if (!Utils::Misc::isPreviewable(ext))
-            continue;
-
         const TorrentInfo::PieceRange extremities = info().filePieces(i);
         const int firstPiecePrio = nativeHandle().piece_priority(extremities.first());
         const int lastPiecePrio = nativeHandle().piece_priority(extremities.last());
@@ -1303,10 +1299,6 @@ void TorrentHandle::setFirstLastPiecePriority(const bool enabled)
         if (filePrio <= 0)
             continue;
 
-        const QString ext = Utils::Fs::fileExtension(filePath(index));
-        if (!Utils::Misc::isPreviewable(ext))
-            continue;
-
         // Determine the priority to set
         const int newPrio = enabled ? 7 : filePrio;
         const TorrentInfo::PieceRange extremities = info().filePieces(index);