Content server: Fix covers not being updated in downloaded copies of some books. Fixes #1699932 [some covers don't show up in reading apps since 3.0 update](https://bugs.launchpad.net/calibre/+bug/1699932)

src/calibre/srv/content.py

@@ -200,6 +200,10 @@ def book_fmt(ctx, rd, library_id, db, book_id, fmt):
     def copy_func(dest):
         db.copy_format_to(book_id, fmt, dest)
         if update_metadata:
+            if not mi.cover_data or not mi.cover_data[-1]:
+                cdata = db.cover(book_id)
+                if cdata:
+                    mi.cover_data = ('jpeg', cdata)
             set_metadata(dest, mi, fmt)
             dest.seek(0)