JavaScript files, such as .js and .mjs should be served with the content-type: "text/javascript" (especially ".mjs" files). The type: "application/javascript" is obsolete.

https://datatracker.ietf.org/doc/draft-ietf-dispatch-javascript-mjs/

The media type registrations herein are divided into two major
categories: the sole media type "text/javascript" which is now in
common usage, and all of the media types that are obsolete.

For both categories, The ECMAScript media types are to be updated to
point to a non-vendor specific standard undated specification of
ECMAScript. In addition, a new file extension of .mjs is to be added
to the list of file extensions with the restriction that it must
correspond to the Module grammar of [ECMA-262]. Finally, the [HTML]
specification is using "text/javascript" as the default media type of
ECMAScript when preparing script tags; therefore, "text/javascript"
has been moved intended usage from OBSOLETE to COMMON.

Chrome enforce this:

To deploy .mjs on the web, your web server needs to be configured to serve files with this extension using the appropriate Content-Type: text/javascript header, as mentioned above. Additionally, you may want to configure your editor to treat .mjs files as .js files to get syntax highlighting. Most modern editors already do this by default.