You'll need two Transport Rules with a disclaimer each:
# Our disclaimer, with HTML and fancy predicates:
$DisclaimerText = @"
<div style="color: #00FF00; font-family: 'Comic-sans'">
We, here at %%Company%% hate the environment, and want YOU to invest in additional storage!
Please take these extra bytes of HTML as a sign of our gratitude towards your continued support of natural resource depletion
*Patent forms*, *legal threats*, *copyright voodo* Trademark pending
</div><div><img src="http://images.malware.biz/cutepuppy.jpg" /></div>
"@
# Create a new transport rule for mails sent from inside the organization and out
New-TransportRule -Name "LegalDisclaimer" -ApplyHtmlDisclaimerText $DisclaimerText -FromScope InOrganization -SentToScope NotInOrganization
The above will apply to all emails sent from your employees to external recipients.
To apply a second disclaimer only in the event that an attachment exists, test if the attachment size exceeds 0 bytes:
# Another disclaimer, with HTML and fancy predicates:
$AttachmentText = @"
<div style="color: #FF0000; font-family: 'Comic-sans'">
We, here at %%Company%% still hate the environment, and want YOU to invest in even MORE additional storage!
Please accept this huge attachment in our course of world domination. Thanks in Advance
*Patent forms*, *legal threats*, *copyright voodo* Trademark pending
</div><div><img src="http://images.malware.biz/cutepuppywithhat.jpg" /></div>
"@
# Create a new transport rule with the same scopes, but for only emails with attachments
New-TransportRule -Name "AttachmentDisclaimer" -AttachmentSizeOver 0 -ApplyHtmlDisclaimerText $AttachmentText -FromScope InOrganization -SentToScope NotInOrganization
But... don't use it, disclaimers are lame