Support local resources for ReactiveHTML components

[ghost]

Environment:

• python=3.9
• folium
• ipywidgets
• jupyterlab
• jupytext
• panel
• pyproj
• sentinelsat
• vtk
• plotly

Happens on multiple machines and on both Windows 10 and Ubuntu.

Description of expected behavior and the observed behavior

Trying to use panel.extend() with external javascript and/or css -files results in a failed import, citing a faulty MIME-type (text/html).

Complete, minimal, self-contained example code that reproduces the issue

import panel as pn
from panel.reactive import ReactiveHTML

class TestClass(ReactiveHTML):
    __javascript__ = ['./index.umd.js'] #point to local file, have tried both relative/absolute paths
    __css__ = ['./index.css'] #point to local file, have tried both relative/absolute paths
    
    _template = """
    <h1 id="hello">Hello World!</hi>
    """
    
pn.extension()

testClassInstance = TestClass()
testClassInstance

Stack traceback and/or browser JavaScript console output

Refused to apply style from 'http://localhost:8888/lab/workspaces/auto-9/tree/index.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.

This does however work when pointing to a CDN for example. I can only reproduce this bug with local files.

[MarcSkovMadsen]

Local files and/ or raw_css would be very nice indeed. I've been missing the raw_css possibility.

[MarcSkovMadsen]

It's a different issue. But we also still lack an api for providing _scripts from a .js file instead of a dictionary. This would make the development process much better as IDE can provide useful feedback.