As it seems, you cant' currently drag and drop a file from the Krusader::Search
dialog. Please file a bug report at https://bugs.kde.org/ for krusader.
As a workaround, double-click on your desired file in the search dialog so it
gets opened in the main krusader window. From there you can drag and drop it
to a thunderbird composer window (but you have to drop it in the upper part,
where the addresses are entered, otherwise just the URL is pasted).
r2ruyu-nana