When I compose an email with message (via mu4e or otherwise), and include an attachment for a LibreOffice document, I can't send the message. When I try to send it, instead of having the normal response - messages indicating that the message is getting sent, followed by the window closing - what happens instead is that the attachment is opened in LibreOffice!
I can send messages without attachments without a problem, or with attachments of a different filetype.