MIME is short for Multipurpose Internet Mail Extensions. It is used to send things by email which are not necessarily simple plain text. (For example, MIME would be used if you were sending a picture of your dog to a friend.)
A MIME digest has each message as an attachment inside the message, along with a summary table of contents.
A plain text digest is a simpler form of digest, which should be readable even in mail readers which don't support MIME. The messages are simply put one after the other into one large text message.
Most modern mail programs do support MIME, so you only need to choose plain text digests if you are having trouble reading the MIME ones.
Note: This option has no effect if you are not receiving mail bunched as digests. (See Section 8.1 for more information on receiving mail as digests.)
To set your digest type using the web interface:
Set it to "MIME" to receive digests in MIME format, or "Plain text" to receive digests in plain text format.
This can also be changed for multiple lists at the same time if you are subscribed to more than one list on the same domain. See Section 10.1 for information about changing settings globally.
To set your digest type using the email interface,
Commands can appear in either the body or the subject of the message. (See Section 3.2 for more information about sending mail commands.)