Hi there, I also get this issue and it is driving me nuts. I’ve been forced back to using Thunderbird because of it, after paying for eM Client licenses.
To be precise, the emails are html emails from any sender sent to my home account using my own server running postfix. I use a filter service to check my emails (boxbe.com) and it wraps html messages with a wrapper. When it does, eM Client displays the emails as attachment.html which have to be manually opened every time, which it does in a web browser.
This problem only happens in eM Client, I checked it with Outlook and Thunderbird on Windows and Linux, and Bluemail and K9 Mail on Android and they all display the email properly with the html content displayed inside the text wrapper and (optionally) the images blocked until I manually choose to download them as normal.
You can reproduce the problem by getting a boxbe account, setting an account up on it and sending yourself an html email to that account. Every other client displays the email except eM Client which has it as an attachment named attachment.html.
(update) Just installed Version 8 (why wasn’t I prompted about that when I selected check for updates?) and unfortunately it’s actually worse in that rather than the attachment being called attachment.html, it’s now called attachment.bin and there is no association with html at all so opening the attachment no longer opens a web browser to at least view the html.