How do I attach image files in Django-summernote?

So I have this Django application with django-summernote, and I'm having trouble uploading image files via the editor. When I click the upload button and choose an image from my computer, I get an error saying "Failed to save attachment".
And in the console, I see this error: "POST /summernote/upload_attachment/ 500". Let me show you my code.

settings.py

INSTALLED_APPS = [
    ...
    'django_summernote',
    ...
]
...
DEBUG = True
...
SUMMERNOTE_CONFIG = {
    'iframe': True,
    'lang' : 'ko-KR',
    'summernote': {
        'width': '100%',
        'height': '450px',
        'placeholder':'First sentence',
        'toolbar': [
            ['style', ['style',]],
            ['font', ['fontsize', 'bold', 'italic', 'strikethrough']], 
            ['color', ['forecolor', ]], 
            ['para', ['ul', 'ol', 'height']],
            ['insert', ['link']],
            ['misc', ['picture', 'fullscreen', 'print', 'help', ]], 
        ],
    },
    'js': (
        'static/summernote-ext-print.js',
    ),
    'js_for_inplace': (
        '/static/summernote-ext-print.js',
    ),
    'css': (
        '//cdnjs.cloudflare.com/ajax/libs/codemirror/5.40.0/theme/base16-dark.min.css',
        '/mvp/static/summernote.css',
    ),
    'css_for_inplace': (
        '//cdnjs.cloudflare.com/ajax/libs/codemirror/5.40.0/theme/base16-dark.min.css',
        '/summernote.css',
    ),
    'codemirror': {
        'theme': 'base16-dark',
        'mode': 'htmlmixed',
        'lineNumbers': 'true',
    },
    'lazy': False,
}

SUMMERNOTE_THEME = 'bs4'
X_FRAME_OPTIONS = 'SAMEORIGIN'
...
MEDIA_URL = '/media/'
MEDIA_ROOT = os.path.join(BASE_DIR, 'media/')

urls.py

urlpatterns = [
    path('summernote/', include('django_summernote.urls')),
]

if settings.DEBUG:
    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)

I don't see what I've done wrong. Everything else except uploading image works fine. BTW, I'm using Django 3.x and Bootstrap 4.x.

+++
If it gives you any further info: I also keep getting these errors in the console as well. Might be related?
Refused to apply style from '<myURL>/summernote.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Failed to load resource: the server responded with a status of 404 () ... summernote-ext-print.js
Failed to load resource: the server responded with a status of 500 () .../summernote/upload_attachment/

This has been making me so frustrated for days. I'd very much appreciate your help. :)