Did they convert you from the older "Zimbra" webmail (web.mail.comcast.net/zimbra/...) to the newer Open-Xchange "Appsuite" (connect.xfinity.com/appsuite/...)? "Zimbra" had that feature, but "Appsuite" does not.
Help with deleting an attachment from an email.
Bruce W is correct, and I have been telling people the same thing: that there is no feature, per se, to delete an attachment from an email that is sent to you. However, since so many people have asked about it, I did some experimenting and I may have come up with a workaround. Drag and drop the message from your Inbox or other folder to the Drafts folder. Once there, locate it in the list and open it. Then click on "Edit" in the toolbar. Now the attachments will appear as they did when the message was composed, and you should see the little trashcan icon to the right of each one. Hit that to delete it. Once you have done that, hit Send to get it back to your Inbox, or whatever other email address you want. When it gets there the attachment will be gone and the rest of the email will be intact.
One drawback-------------when you have the edited email back in your Inbox, you will show in the header as the sender, rather than whoever really sent it to you.
Yeah I know-----bit of a job, but it does seem to work. Hardly something I'd want to do on a frequent basis though.
ETA: As of today, 12/14/2017, I have done further research into this issue. I have looked at accounts I have from Gmail, Yahoo, Outlook.com, and iCloud---------------none of them has the ability to delete an attachment from a message in the Inbox folder. So this is nothing peculiar to Comcast email. The same applies if you are using a desktop client like Mac Mail or Thunderbird, and in fact, doing so adds yet another step or two to what I posted above. In all cases, you have to resort to some kind of workaround to get rid of an attachment while keeping the email itself.
I have found only one exception and that is my Mail.com account (1&1). The attachments appear as usual above the body of the email, and there is an X you can click on to delete it. It will ask if you are sure, but you can delete it from the Inbox with no further bother.