Hi! I tested a workaround with the following steps on both local 15.0 and 15.5.4 instances:

1. I extracted the document-formats.js from the dedicated jar xwiki-platform-legacy-office-importer.jar from WEB-INF/lib (I also tried with just copying the document-formats.js locally from here and here )
2. Modified the document-formats.js, HTML related section by changing the value "inputFamily": "WEB" to "inputFamily": "TEXT" and saved it.

 "name": "HTML",
    "extensions": [
      "html"
    ],
    "mediaType": "text/html",
    "inputFamily": "TEXT",
    "storeProperties": {
      "SPREADSHEET": {
        "FilterName": "HTML (StarCalc)"
      }, 

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1. Replaced the document-formats.js in the xwiki-platform-legacy-office-importer.jar from WEB-INF/lib (I also tried with the option to place the document-formats.js in the \xwiki-platform-distribution-flavor-jetty-hsqldb-version\webapps\xwiki\WEB-INF\classes without modifying the xwiki-platform-legacy-office-importer.jar)
2. Restarted the local wiki (which has an active Office server)
3. Tried to import a local HTML file using the Import Office File option.

Expected result: the content from the file is imported as content of the page

Actual result: it fails to import.