Hotmail, Yahoo & Gmail strips attachments, images and html/rtf formatting for calendar.
Outlook may not support HTML, Rich Text, and attachments when you are using a POP or Internet email account like Hotmail, Live, Gmail, Yahoo, or Outlook.com. Outlook supports HTML and Rich Text formatting for IMAP and Exchange email accounts.
If your email account is not configured as IMAP or Exchange, and calendar appointments/meetings are in plain text, change your email account to either IMAP or Exchange account configuration. See support article for Configure Outlook with Outlook.com, Hotmail, Gmail, Office 365, Yahoo, AOL or iCloud
and see Microsoft support article for Outlook Email Setup
to change your account type. Please contact your network administrator if you do not know your company's email server settings.