I really don't think you'll ever find such files on LW users... 'cose LW by default does not share such files. (may be other program users)

Here is the complete list of shared extensions in LW by default:
asx;html;htm;xml;txt;pdf;ps;rtf;
doc;tex;mp3;mp4;wav;
wax;au;aif;aiff;ra;ram;wma;
wm;wmv;mp2v;mlv;mpa;mpv2;
mid;midi;rmi;aifc;snd;flac;fla;
mpg;mpeg;asf;qt;mov;avi;mpe;
swf;dcr;gif;jpg;jpeg;jpe;png;tif;
tiff;exe;zip;gz;gzip;hqx;tar;
tgz;z;rmj;lqt;rar;ace;sit;smi;
img;ogg;rm;bin;dmg;jve;nsv;
med;mod;7z;iso;lwtp;pmf;m4a;
idx;bz2;sea;pf;arc;arj;bz;
tbz;mime;taz;ua;toast;lit;
rpm;deb;pkg;sxw;l6t;srt;sub;
idx;mkv;ogm;shn;flac;fla;dvi;
rmvp;kar;cdg;ccd;cue;c;h;
m;java;jar;pl;py;pyc;pyo;pyz

So only if someone ads the .sis extension here (on their computers) than may be you'll be lucky...