Add a Share button. Works with Gmail, Drive, Dropbox...

Special handling for Bluetooth; thanks to Roger Keays for http://www.ninthavenue.com.au/how-to-remove-the-facebook-android-sharing-intent

No thanks to the Android team for https://android.googlesource.com/platform/packages/apps/Bluetooth/+/android-5.0.0_r2/src/com/android/bluetooth/opp/Constants.java#204

...also for requiring SD read permissions to read received files; i.e. you get a file:// URL not content://. Dropbox app has the same problem.