I have an app that stores it's images on the external storage. This have been working great up until I tried the new M preview.
I use picasso (I even tried Ion) to load the images and I get the images with the "file:///mnt/sdcard/appname/image1.jpg" URI.
I don't get any errors at all, but I'm guessing that M has changed the permissions to read from external storage. I have tried googling but I come up empty.
Writing the images to the external storage works just as normal by the way.