As per the example below, the preview for the MHT file type doesn't seem to be loaded with the correct formatting:

Issue Attachments Example

For example, by clicking on an image or a PDF, the preview is correctly displayed before downloading the file:

PDF Attachment Preview

However, when clicking on the MHT file:

MHT File Preview

The preview displays the code instead of the formatted file. The expected behavior would be displaying the file as it happens when opening through a browser:

MHT File displayed on Browser

As a side note, this seems to be a problem with HTML attached files as well.