Streak doesn't support by-recipient attachments. Adding an attachment to your mail merge draft will add that attachment for all included recipients.
Working around this
You can host your attachments publicly to a tool like Dropbox or Google Drive and add the public URL to a column in your pipeline.
You can then use that column's variable in the body of your message. This means each recipient will receive a unique link (as specified in the column) in the body of your email.