I just reset my device (Lumia 830, Windows 10 Mobile build 10586.494) and after I installed WhatsApp I noticed that when I wanted to send a picture or a video I could not see the thumbnails from the photos took or saved before the reset, I'm just able to see the thumbnails of those pictures that were taken after the reset. When I go to the Photos app everything seems normal.
https://1drv.ms/i/s!AkzaJMzengqoq9ECl810QXz5hkPHGw
https://1drv.ms/i/s!AkzaJMzengqoq9ECl810QXz5hkPHGw