My email default program is Google Chrome. When trying to send a LibreOffice file the pop-up advises " There is no program associated …etc. I have no problem when sending other files; text or photos.
Google Chrome is an internet browser. If you manage your email in your browser calling some web page of your mail provider (Google?), then all you need to do is adding an attachment to your email. The type of file should not matter at all. Click some button to add an attachment and browse to your file (office document, picture or any other file). That’s it.
If you use the Calc menu item File > Send > Email document then LibreOffice passes it to the system default email program. If you do not have an email program registered with the operating system then you will get some pop-up. Same if you click a mailto: link unless it is within your browser in the email page.
How do you add pictures as attachments? Do it the same way, as Villeroy suggests.