diff --git a/src/calibre/db/backend.py b/src/calibre/db/backend.py
index b1bbd77680..96557722ef 100644
--- a/src/calibre/db/backend.py
+++ b/src/calibre/db/backend.py
@@ -1889,7 +1889,7 @@ class DB:
             os.makedirs(tpath)
         update_paths_in_db()
 
-    def iter_extra_files(self, book_id, book_path, formats_field):
+    def iter_extra_files(self, book_id, book_path, formats_field, yield_paths=False):
         known_files = {COVER_FILE_NAME, METADATA_FILE_NAME}
         for fmt in formats_field.for_book(book_id, default_value=()):
             fname = formats_field.format_fname(book_id, fmt)
@@ -1904,24 +1904,35 @@ class DB:
                     relpath = os.path.relpath(path, full_book_path)
                     relpath = relpath.replace(os.sep, '/')
                     if relpath not in known_files:
-                        try:
-                            src = open(path, 'rb')
-                        except OSError:
-                            if iswindows:
-                                time.sleep(1)
-                            src = open(path, 'rb')
-                        with src:
-                            yield relpath, src, os.path.getmtime(path)
+                        mtime = os.path.getmtime(path)
+                        if yield_paths:
+                            yield relpath, path, mtime
+                        else:
+                            try:
+                                src = open(path, 'rb')
+                            except OSError:
+                                if iswindows:
+                                    time.sleep(1)
+                                src = open(path, 'rb')
+                            with src:
+                                yield relpath, src, mtime
 
     def add_extra_file(self, relpath, stream, book_path):
         dest = os.path.abspath(os.path.join(self.library_path, book_path, relpath))
-        try:
-            d = open(dest, 'wb')
-        except OSError:
-            os.makedirs(os.path.dirname(dest), exist_ok=True)
-            d = open(dest, 'wb')
-        with d:
-            shutil.copyfileobj(stream, d)
+        if isinstance(stream, str):
+            try:
+                shutil.copy2(stream, dest)
+            except FileNotFoundError:
+                os.makedirs(os.path.dirname(dest), exist_ok=True)
+                shutil.copy2(stream, dest)
+        else:
+            try:
+                d = open(dest, 'wb')
+            except FileNotFoundError:
+                os.makedirs(os.path.dirname(dest), exist_ok=True)
+                d = open(dest, 'wb')
+            with d:
+                shutil.copyfileobj(stream, d)
 
     def write_backup(self, path, raw):
         path = os.path.abspath(os.path.join(self.library_path, path, METADATA_FILE_NAME))
diff --git a/src/calibre/db/copy_to_library.py b/src/calibre/db/copy_to_library.py
index c9e4ee22f9..6488ec636b 100644
--- a/src/calibre/db/copy_to_library.py
+++ b/src/calibre/db/copy_to_library.py
@@ -102,6 +102,12 @@ def copy_one_book(
         new_book_id = newdb.add_books(
             [(mi, format_map)], add_duplicates=True, apply_import_tags=tweaks['add_new_book_tags_when_importing_books'],
             preserve_uuid=preserve_uuid, run_hooks=False)[0][0]
+        bp = db.field_for('path', book_id)
+        if bp:
+            for (relpath, src_path, mtime) in db.backend.iter_extra_files(book_id, bp, db.fields['formats'], yield_paths=True):
+                nbp = newdb.field_for('path', book_id)
+                if nbp:
+                    newdb.backend.add_extra_file(relpath, src_path, nbp)
         postprocess_copy(book_id, new_book_id, new_authors, db, newdb, identical_books_data, duplicate_action)
         return_data['new_book_id'] = new_book_id
         return return_data
diff --git a/src/calibre/db/tests/add_remove.py b/src/calibre/db/tests/add_remove.py
index 33e069e5bf..8e08f58146 100644
--- a/src/calibre/db/tests/add_remove.py
+++ b/src/calibre/db/tests/add_remove.py
@@ -377,6 +377,12 @@ class AddRemoveTest(BaseTest):
             a(type='highlight', highlighted_text='text2', uuid='2', seq=3, notes='notes2 some word changed again'),
         ]
         src_db.set_annotations_for_book(1, 'FMT1', annot_list)
+        bookdir = os.path.dirname(src_db.format_abspath(1, '__COVER_INTERNAL__'))
+        with open(os.path.join(bookdir, 'exf'), 'w') as f:
+            f.write('exf')
+        os.mkdir(os.path.join(bookdir, 'sub'))
+        with open(os.path.join(bookdir, 'sub', 'recurse'), 'w') as f:
+            f.write('recurse')
 
         def make_rdata(book_id=1, new_book_id=None, action='add'):
             return {
@@ -416,5 +422,8 @@ class AddRemoveTest(BaseTest):
         for new_book_id in (1, 4, 5):
             self.assertEqual(dest_db.format(new_book_id, 'FMT1'), b'replaced')
         self.assertEqual(dest_db.format(rdata['new_book_id'], 'FMT1'), b'second-round')
+        bookdir = os.path.dirname(dest_db.format_abspath(1, '__COVER_INTERNAL__'))
+        self.assertEqual('exf', open(os.path.join(bookdir, 'exf')).read())
+        self.assertEqual('recurse', open(os.path.join(bookdir, 'sub', 'recurse')).read())
 
     # }}}
diff --git a/src/calibre/gui2/preferences/create_custom_column.py b/src/calibre/gui2/preferences/create_custom_column.py
index 01b22b2377..627f02319b 100644
--- a/src/calibre/gui2/preferences/create_custom_column.py
+++ b/src/calibre/gui2/preferences/create_custom_column.py
@@ -459,7 +459,7 @@ class CreateCustomColumn(QDialog):
         l = QHBoxLayout()
         self.composite_in_comments_box = cmc = QCheckBox(_("Show with comments in Book details"))
         cmc.setToolTip('<p>' + _('If you check this box then the column contents '
-                                 'will show in the Comments section in Book details. '
+                                 'will show in the Comments section in the Book details. '
                                  'You can indicate whether not to have a header or '
                                  'to put a header above the column. If you want a '
                                  "header beside the data, don't check this box. "