This contribution will allow you to send email to customers with an attachment included (from the admin panel) , there are no filetype or size limitations (you can modify the code to allow limits).
Only the store administrator will be able to use this option.
This was tested with .rar, .doc, .xls and .pdf files without problems, should work for others too. Hotmail and Gmail accounts received and downloaded the attachment without problems.
If you dont want to include an attachment just leave the file upload field blank (dont choose a file). The email will be sent without attachment as it does by default.