Solution how to import contents of a .txt file into a body field?
I believe you can accomplish this with Feeds Tamper and a custom plugin.

Take one of the existing text plugins and clone it to a new one.  Then inside the plugin callback, take the cell value (your text file name), build up a proper path, do a file_get_contents(), and stuff the contents into the result.

If you end up going this route, it may be a decent patch to Feeds Tamper for inclusion in the project itself.