Is the document correctly identified in the message body for a handler to be determined. The file name appears to indicate that the attachment may be a word document. However unless the media type is correctly set in the email, there is no definitive file type to recommend an appropriate helper application.

If it is a word document, then the media type according to Mozilla should be application/vnd.openxmlformats-officedocument.wordprocessingml.document

The source can be accessed by using the Ctrl+U shortcut. You are looking for a block that defines the media. It looks like the following which is a PDF attachment description I extracted from a mail in my inbox.

Content-Type: application/pdf; name="Slip.pdf"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="Slip.pdf"

I have seen a lot of script generated emails where the software authors simply failed to grasp even what the media type is (read Windows/ Microsoft only developers) They tend to use a generic binary media type for the attachment so the only option offered is to save the file and can not comprehend that the file extension is irrelevant outside of Windows.

The document is correctly identified in the message body for a handler, so that can be eliminated as a issue.

The problem appears to be that the handlers are missing and I don't know how to get them back. See screenshot in original post, there are no options other than Thunderbird, where there should be a list of applications.