Splitting this off from the combination issue #3075:

• Test downloading files, and sharing images (i.e. our shareImage.) There's a note about file:// URLs which I think isn't an issue for downloading, but may be for our sharing; and I don't 100% understand what it's saying anyway. Some grepping finds file:// URLs are used in several ways in rn-fetch-blob, which in turn we use in downloadFile, which is used in those two places.

This is about a change in Android system behavior, which took effect for us when we increased targetSdkVersion to 26 from 22.