properly reset header for preview fragments

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

src/main/java/com/owncloud/android/ui/fragment/OCFileListFragment.java

@@ -1001,10 +1001,13 @@ public class OCFileListFragment extends ExtendedListFragment implements
                         ((FileDisplayActivity) mContainerActivity).startContactListFragment(file);
                     } else if (PreviewTextFileFragment.canBePreviewed(file)) {
                         setFabVisible(false);
+                        resetHeaderScrollingState();
                         ((FileDisplayActivity) mContainerActivity).startTextPreview(file, false);
                     } else if (file.isDown()) {
                         if (PreviewMediaFragment.canBePreviewed(file)) {
                             // media preview
+                            setFabVisible(false);
+                            resetHeaderScrollingState();
                             ((FileDisplayActivity) mContainerActivity).startMediaPreview(file, 0, true, true, false);
                         } else {
                             mContainerActivity.getFileOperationsHelper().openFile(file);
@@ -1018,6 +1021,8 @@ public class OCFileListFragment extends ExtendedListFragment implements
                         if (PreviewMediaFragment.canBePreviewed(file) && account.getServer().getVersion()
                             .isMediaStreamingSupported() && !file.isEncrypted()) {
                             // stream media preview on >= NC14
+                            setFabVisible(false);
+                            resetHeaderScrollingState();
                             ((FileDisplayActivity) mContainerActivity).startMediaPreview(file, 0, true, true, true);
                         } else if (FileMenuFilter.isEditorAvailable(requireContext().getContentResolver(),
                                                                     accountManager.getUser(),