Based on what you said it's hard to tell you what the problem is. We need more details about what is or is not happening when you attempt to add an attachment.
Are you using Xfinity Connect or an email client?
If a client which one? If Xfinity Connect which version-full or lite? To find out click Preferences. General icon. What does it say next to Version?
Is the Add Attachment grayed out? If not, what happens after you click it?
Anything else we need to know?
Comcast employees must be authorized to post in the forum in an official capacity. Employees posting here have their names in red and are designated as employees. Names not in red are customers.
This is done to protect customers and for assurance that they are dealing with a Comcast employee. Non-Authorized Employees are allowed to post but cannot state they are employees nor can they allude to being employees.