Errer when generating with dir as a source path if there is a dir inside it

[Solander]

My folder structure is as follows:
pics/
test.jpg
lala.jpeg
inner/
another.jpg
picture.jpeg

It works to generate thumbnails for pics/inner/ but not for pics/. The only error I get in return, is "Unsupported file format". Would be nice with a more descriptive message and also the possibility of digging down when generating. Or maybe the option of only generate for the files in the directory and discard inner directories.