Hi Jonathan. Someone who can read the source code, like Sam Berlin and others, could tell you. There's a thread in the Future Requests forum about Tip of the Day. I think such a list would be a useful tip of the day--a post there would be noticed.

In the meantime, all the current file types are listed in the Options->Sharing. I'd guess the file type searches would be a sub set of those extensions, but knowing which ones would be a matter of trial and error. Here's the list I can see, but I know some .zip or .sit files could be documents or programs--others are more obvious.

asx;html;htm;xml;txt;pdf;ps;rtf;doc;tex;mp3;mp4;wa v;wax;au;aif;aiff;ra;ram;wma;wm;wmv;mp2v;mlv;mpa;m pv2;mid;midi;rmi;aifc;snd;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;l it;rpm;sxw