This is reproducible on confluence.atlassian.com - the attached Word document has been generated from there.
Doing a head on the file results in the following:
Subject: Exported From Confluence
Content-Type: text/html; charset=UTF-8
Basically what is going on is that this is a multi-part email which Microsoft Word is able to display. It is not a real binary .doc file which can be opened by other tools which can open Word documents.
If a Confluence page that contains image being exported to Word and when this Word file is attached again into the Confluence page, the image will not be able to rendered properly. Most probably due to the file not being an actual .doc file:
Open the exported file with Microsoft Word and use the Save As.. option to save the file as .doc or .docx file