Is it a bug that exporting does not work, unless I manually type the file extension?

If I click File -> Export..., then change the format to PNG images, and then click the "Export..." button, MuseScore shows a file dialogue. At its bottom-right corner, there is the file type drop-down. It's "PDF files" when exporting as PDF, and "PNG images" when exporting as PNG. That is the only entry in the drop-down, so I cannot change the type. Since it already knows the file type and I cannot even change it, I thought manually typing the file extension was not necessary.

There is the Name field at the top, and it is empty. If I type only the file name, for example, sample and then click "Save", nothing happens. If I type a wrong extension like sample.dummy, nothing happens either. Only when I typed sample.png, export works. I have tried both PDF and PNG, and the behaviour was the same.

Is this a bug? If it is intended, why can't it attach the default extension, when the type has already been selected? For example, Windows Notepad automatically adds ".txt" if the chosen file type is Text.