Help .htm attachment not opening in new phone

[htm attachment]

Huawei Y5

Hi

New phone. I get an email with a .htm attachment.
When I open the attachment, I get:
"There was an error while decoding the message"
I get the same error when opening the message in Chrome, Android HTML viewer, and Opera.
The same .htm message, which I regularly get, opened with no problems on my old Samsung phone.
The message opens correctly on the desktop.
I get same error message when I try and open it in the gmail app.

Any suggestions would be appreciated.