Not working on voila

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When trying to use sage_combinat_widgets on voila, the following message appears on the browser console:

The resource from “http://localhost:8888/voila/sage-combinat-widgets.js” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff).

(although there is no such file in the code)

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Steps to reproduce

1. Dockerfile
   FROM sagemath/sagemath:9.1.rc5
   RUN sage -pip install --upgrade jupyter_core jupyter_server nbformat nbclient nbconvert pygments voila
   RUN sage -pip install --upgrade ipywidgets sage_combinat_widgets
   ENTRYPOINT /usr/bin/sage -n jupyter --ip=0.0.0.0 --port=8888

2. docker build, and docker run with -ti -p8888:8888

3. Open a Sagemath notebook, and type:

from sage_combinat_widgets import *
GridViewWidget(Partition([4,3]))

4. Try to voila