[url_image_preview] Saving jpg locally does not work in flatpak

Versions

• OS: Linux
• GTK+ Version: 3.24.13
• PyGObject Version: 3.34.0
• python-nbxmpp Version: 0.6.10
• Gajim Version: 1.1.3

Traceback

Traceback (most recent call last):
  File "/app/lib/python3.7/site-packages/gajim/gtk/filechoosers.py", line 49, in _on_response
    accept_cb(dialog.get_filename())
  File "/app/plugins/url_image_preview/url_image_preview.py", line 657, in on_ok
    shutil.copy(filepath, target_path)
  File "/usr/lib/python3.7/shutil.py", line 249, in copy
    copymode(src, dst, follow_symlinks=follow_symlinks)
  File "/usr/lib/python3.7/shutil.py", line 144, in copymode
    chmod_func(dst, stat.S_IMODE(st.st_mode))
OSError: [Errno 38] Die angeforderte Funktion ist nicht implementiert: '/run/user/1000/doc/f305509f/test.jpg'

Steps to reproduce the problem

1. send picture in 1 to 1 chat or group chat.
2. on the receiving system (Linux Mint 19.2 Cinammon) chat window, saving a picture to local SSD disk in 1 to 1 chat or group chat leads to the error message. I've tried 3 different resolutions/formats However, file has been saved and opens without problems.