Unable to load .mjs file and run tests with Mocha - mocha.js

Question

I am unable to load an .mjs file and run tests with mocha.
I have tried mocha v.7.2.0 with flag --experimental-modules and v.10.0.0 without flag.
I am using node v.14.15.4.
The import statement that I have in my code to test is:
import * as XLSX from 'xlsx/xlsx.mjs';
NB: this import statement works 100% fine while building my app with Webpack.
The error I get when I try to run unit tests on this code with Mocha is:
(node:5452) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use node --trace-warnings ... to show where the warning was created)
C:\Users......webapp\node_modules\xlsx\xlsx.mjs:81
export { set_cptable };
^^^^^^
SyntaxError: Unexpected token 'export' at wrapSafe (internal/modules/cjs/loader.js:979:16)
My mocha invocation is a vanilla mocha \"src/**/*.spec.js\", which I call through an npm task; as I mentioned before, flag --experimental-modules does not help.
My .babelrc, which (I suspect) is used to transpile my code, is the following:
{
"presets": [
"#babel/preset-env",
"#babel/preset-react"
],
"plugins": [
"#babel/plugin-transform-runtime",
"react-hot-loader/babel",
["module-resolver", {
"alias": {
"#src": "./src"
}
}]
],
"env": {
"coverage": {
"plugins": [
"istanbul"
]
}
}
}
What can I do to solve this problem?