Export file is actually a bitmap (bmp) file

When saved using the "Save as image" option the output is actually a bitmap (PC bitmap, Windows 3.x):

$ file test.png 
test.png: PC bitmap, Windows 3.x format, 300 x 300 x 24, image size 270000, resolution 100 x 100 px/m, cbSize 270054, bits offset 54

The DefaultExt is set to .png so when saving if no extension is given it saves as a .png file, which does not match the data the file has.