Suggest as you are still getting the 2 attachments “even when sending plain text emails from Gmail webmail”, then I would contact official support via lodging a ticket to have your emails investigated via the VIP support page.
If you have run out of your 12 months support you can get another year support via the following page VIP Support Extension | eM Client
When you login and lodge the ticket “attach an example email received (.eml) message file via right clicking in the body of the email and “save as” to a folder on your computer. Also let them know your eM Client version and Win or Mac OS version.
Apart from that if you happy to try the latest eM Client 11 Beta see if that makes any difference.
Note: If you do try the Beta, “make a backup first” via “Menu / Backup” incase you need to restore. You can see when that’s completed in Show Operations via the dropdown on the right of Refresh. Also close eM Client before updating.