Extension .17 is a genuine file extension and this year's date can also be shown as .17. Consequently I find that when files such as email (.eml) and text (.txt) are saved with the date such as 11.4.17 at the end it is mistaken as a file extension and my saved file finishes as a gobbledegook text type file called a 17 file. This can be rectified by renaming to the correct extension and prevented by adding the date after it is saved. The glitch only seems to occur when "hide extensions for known file types " is checked/ ticked in Folder Options. I have XP and Outlook Express but I find it can happen in Windows 7 with Outlook. I can't speak for all OS's but I thought this information could help prevent some puzzling and head scratching if it happens to you.