Attaching files is not supported in Messages. There are several good reasons why attaching files to a bulk email campaign is a bad idea.
Bulk emails sent with attachments are likely to be filtered out by spam filters at the ISP level.
The performance of a mailing with attachments is dramatically slower, as the size of every single email in the campaign is drastically increased.
Attachments can't be updated after they're sent. If an attached file has a typo or needs to be updated, you’d have to send a new message. With a linked file, you can always update the file saved on your website. Anybody who clicks a link your email back to the site would download the latest version of the file.
An attached file gives you no record of how many people opened it. When you create a link in a message to a file, on the other hand, you will have a record of how many people click on the link and, therefore, how many people open the file.
Instead of adding attachments, you can link a file from Resources or File Manager to an eNotice. On the reader's end this will function exactly like an attachment, and the file is only sent to those recipients who specifically request it. To find out how to link a file to a message, refer to the article Link to files in Messages.