prevent receiving (as share with target) file URIs

as Element (Matrix client) demonstrated again file URIs are unnecessarily dangerous. On Android 7+ there is no good reason to process them anymore

src/main/java/eu/siacs/conversations/persistance/FileBackend.java

@@ -370,29 +370,35 @@ public class FileBackend {
         }
     }
 
-    public static boolean weOwnFile(final Uri uri) {
+    public static boolean dangerousFile(final Uri uri) {
-        if (uri == null || !ContentResolver.SCHEME_FILE.equals(uri.getScheme())) {
+        if (uri == null || Strings.isNullOrEmpty(uri.getScheme())) {
-            return false;
+            return true;
-        } else {
-            return weOwnFileLollipop(uri);
         }
+        if (ContentResolver.SCHEME_FILE.equals(uri.getScheme())) {
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+                // On Android 7 (and apps that target 7) it is now longer possible to share files
+                // with a file scheme. By now you should probably not be running apps that target
+                // anything less than 7 any more
+                return true;
+            } else {
+                return isFileOwnedByProcess(uri);
+            }
+        }
+        return false;
     }
 
-    private static boolean weOwnFileLollipop(final Uri uri) {
+    private static boolean isFileOwnedByProcess(final Uri uri) {
         final String path = uri.getPath();
         if (path == null) {
-            return false;
+            return true;
         }
-        try {
+        try (final var pfd =
-            File file = new File(path);
+                ParcelFileDescriptor.open(new File(path), ParcelFileDescriptor.MODE_READ_ONLY)) {
-            FileDescriptor fd =
+            final FileDescriptor fd = pfd.getFileDescriptor();
-                    ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY)
+            final StructStat st = Os.fstat(fd);
-                            .getFileDescriptor();
-            StructStat st = Os.fstat(fd);
             return st.st_uid == android.os.Process.myUid();
-        } catch (FileNotFoundException e) {
+        } catch (final Exception e) {
-            return false;
+            // when in doubt. better safe than sorry
-        } catch (Exception e) {
             return true;
         }
     }

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -2619,10 +2619,10 @@ public class ConversationFragment extends XmppFragment
         final Iterator<Uri> iterator = uris.iterator();
         while (iterator.hasNext()) {
             final Uri uri = iterator.next();
-            if (FileBackend.weOwnFile(uri)) {
+            if (FileBackend.dangerousFile(uri)) {
                 iterator.remove();
                 Toast.makeText(
-                                getActivity(),
+                                requireActivity(),
                                 R.string.security_violation_not_attaching_file,
                                 Toast.LENGTH_SHORT)
                         .show();