I think this depends on what c-client version you have used to compile PHP with. I downloaded the latest I could find (
#define TYPETEXT 0              /* unformatted text */
#define TYPEMULTIPART 1         /* multiple part */
#define TYPEMESSAGE 2           /* encapsulated message */
#define TYPEAPPLICATION 3       /* application data */
#define TYPEAUDIO 4             /* audio */
#define TYPEIMAGE 5             /* static image */
#define TYPEVIDEO 6             /* video */
#define TYPEMODEL 7             /* model */
#define TYPEOTHER 8             /* unknown */
#define TYPEMAX 15              /* maximum type code */

And on the manual page http://php.net/imap_fetchstructure there is an user note containing reply from Mark:

> To allow applications to support types and encodings that are unknown
> to c-client, c-client will automatically add a (limited) number of
> unknown types and encodings to its tables before resorting to
> TYPEOTHER and ENCOTHER.  The names of these added types and encodings
> are available in the body_types[] and body_encodings[] arrays.
>
> Put another way, TYPEOTHER and ENCOTHER are only used if c-client is
> overflowed with unknown types and/or encodings.

There's already a FR about this, see bug #43061 (I'm closing this one in favor of that)