Quote:
SYMPTOMS
When you attempt to insert attachments in a Microsoft Outlook e-mail message, you cannot select multiple files from the Insert File dialog box.

CAUSE
This is by design when you are using Microsoft Word as your default e-mail editor.

RESOLUTION
Select the attachment files one at a time from Insert File dialog box.

-or-

Select multiple files from Windows Explorer and drag them onto the message.

-or-

Change your default e-mail editor to Microsoft Outlook Rich Text (RTF).