Mac Mail attachments not attaching!

My Dad (bless him) has a very strange problem on his new iMac.

When he clicks on New Mail and then attachments he will then select a file to attach, then Choose File it will not attach the attachment.

Nothing appears, and he sends the email to me and there is definitely no attachment.

I have been round to his and tried loads of things and it is definitely not working.

Anybody got any ideas how to solve this problem?

Thanks
PD

What kind of file is the attachment? Just curious. I have never had that issue and am always sending pictures to close friends and family as attachments.