2.40 seems to not allow me to open .wma files via the "Change Directory" icon or via the "File" drop down menu. The window does not show the .wma files in a folder, nor does it seem to have a way to change the acceptable file types from this window. I do have .wma as a valid file type in the Tools->Options->Tags->Restrict Files to box.
Playing around I discovered I can still select files from Windows Explorer and drag them into MP3Tag and edit the tags.
2.39 worked fine for me, and I installed over the top of it.