Steps to reproduce
The following works in Firefox; I did not check in Chrome yet.
Configure your browser to ask you where to save downloaded files.
Go to “☰ → Export … → Formatted”, starting from the context menu of any node in a Dynalist document.
Click “Download as file” and wait for the popup to open that asks you where to save it.
The download popup should identify the document as “HTML document”.
The document is identified in the download popup as “plain text document”. That’s the wrong MIME type.
Latest Firefox on Ubuntu Linux 19.10.
In Firefox, the behavior described above has the following undesired effect: when clicking on the downloaded file in the browser’s list of finished downloads, Firefox will open the file in a text editor rather than in a browser tab.