1.) Plain Text:- This format is supported by all the email clients. This format does not support any formatting i.e. no bold, italics, colored fonts, etc. will not be supported.

2.) Rich Text Format (RTF):- The RTF is only supported by MS Outlook and MS Exchange clients. When messages are sent from Outlook in RTF, they are converted to HTML format in order to maintain the formatting. When a file is attached in a mail in RTF format, it gets affixed as an icon in the message body, as shown in the image below: 

3.) HTML Format:- The HTML format is the default message format of Outlook. The messages sent in this format are received in exactly the same formatting. The message is received in the same form in which it was sent. Whenever a file is affixed in the mail, it is shown in the Attached section and not in the message body, as shown in the figure below: 

How to Affix Attachments in the Attached Tab?

In case, you want to see the affixed attachments in the Attached section, you can do so by changing the formatting of the emails.

1.) For All the Messages:- If you want to affix the files in the Attached section for all the mails you send, follow th below-mentioned steps to do so:

• Click on File option. 
• From the drop-down menu, select Options. 
• Select the Mail option. 
• In the Compose messages in this format section, select the format as HTML and click on OK.