Users working with Monthly Update, or Monthly Update (Targeted) channels, have enoucntered a failure when attaching documents to email messages.
Microsoft has acknowledged this issue as a bug in the mentioned update channels.
To workaround this failure, do one of the following:
Select the document(s) you want to attach, right-click and select Add as Attachment. This adds the document(s) to the current email you are composing.
- or -
Hold the Alt key while dragging the document(s) you want to attach to your Inbox email list. This creates a new email message with the document(s) attached to it.