Description:
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With the advent of ES6 modules, the .jsm file extension is increasingly been used for ES6 module files.

I propose that support be added for this extension to the inbuilt development web server such that `*.jsm` files return `application/javascript`.

Test script:
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N/A

Expected result:
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*.jsm files return a content-type of "application/javascript".

Actual result:
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*.jsm files return a content-type of "application/octet-stream".