Open a new writer document, type random text, and then "Save as" a filename. By default "Automatic File Name Extension" is unchecked, so if you fail to notice (as I have now a number of times) you end up saving a file with the right name, but with no file extension instead of "Filename.odt". 
Now save the file as a proper .odt Writer file. Now try to export it as a PDF by pressing the button in the toolbar. The filename is "Filename.odt", and "Automatic Filename Extension" is turned off. Again, if you don't notice, you try to export, but the resultant PDF is called "Filename.odt", not "Filename.pdf", and you get an error message telling you that the file already exists, do you want to over-write it?
It seems to me that in all these dialogues, "Automatic File Name Extension" should default to "On".