I will just post this in case this helps someone. I looked more closely at this and even went to the extent of installing Thunderbird to see if it was Mail or possibly an OS issue.
What I found in my case was that it was a .tiff issue. When I installed and setup Thunderbird it worked for all the attachments but .tiff images.
When I tried other image types they appeared to work. I know this is probably happening to all file types but I thought I would add this just in case it helps someone.
What appears to have happened, is historically when people use the select tool in the preview application and copy and paste into the mail app, it seems to convert it into a .tiff file. Prior to Sierra, mail then supported .tiff files inline with the email.
In the new version when you paste from preview it pastes it as a .png file (much more sensible) however the new version of mail appears to no longer support viewing .tiff files inline (and probably other file types).
I hope that has helped someone, I searched through the release log of SierraOS and found nothing that indicated any change in this area but it sure seems to have caused problems for me.