When you are viewing the details of a specific bug, you can choose to affix an attachment to that bug report.
Go to Add Attachment at the bottom of the page. Browse for the file to be attached. Add an optional description of the attachment and click Submit.
You can also add an attachment at the same time you are adding a new bug.
Attachments appear under the bug's details and list Who added the attachment, the Filename of the attachment, and a Description of the attachment.
Clicking on a Filename will open up that attachment.