Sent email attachments showing as code

I've just sent off a lot of emails all with attachments. When i checked my sent mail folder about a third of the emails are showing the attachment as code. Is that how it would be received by the other person? Every mail used the same attachment and file opens fine and doesn't seem to be corrupted in any way. They are all work emails so it is embarrassing to send a colleague/customer an email with vast amounts of gibberish.

I'd be very grateful for any advice to rectify this problem.