Thanks for your help.
To update this for anyone else reading, here is a simplified version.
The relevant file is (most likely) here: C:\Program Files (x86)\J River\Media Center 19\Data\Default Resources\FileAssociations.xml
There are two lines that need amending (per each additional file type addition). Look for these:
<!-- Descriptions -->
<DescriptionTypes>
<Audio>WAV Audio (wav,w64,raw,l16)....
Find your media type and add the file extention. For me, adding docx meant this correction:
<Data>MS Office Files (doc,docx,xls,csv,ppt,mde,mdb);.......
The second part is:
<!-- Mime -->
<MimeAssociations>
The <data> portion needs the following line (see the last line below) included:
<Data>
<Item Name="c">text/c</Item>
<Item Name="cpp">text/cpp</Item>
<Item Name="csv">text/csv</Item>
<Item Name="doc">application/msword</Item>
<Item Name="docx">application/msword</Item>
Save, close and restart MC (if necessary). Check your auto import and the new file type should show up. Run your import and it should work.
Thanks