wc3converter fix BLP as iformat for converter

[tdauth]

./wc3converter --oformat png ./bla.blp --iformat blp ./wowr3.4/ReplaceableTextures/CommandButtons/BTNApache.blp
Path "./wowr3.4/ReplaceableTextures/CommandButtons/BTNApache.blp" has not extension "blp".
No valid input files.