"And to his father he sent like this: ten donkeys carrying from the good of Mitzrayim and ten she-donkeys carrying corn and bread and food (mazon) to his father, for the way." (45:23)
The Talmud Yerushalmi (Nedarim 6:1) comments, regarding this verse: "Since it already says corn and bread, what does "mazon" teach us? Rather, it teaches us that all food items are called "mazon"... [aside for salt and water]"
Nonetheless, we are well aware that the blessing of mezonot prior to eating was not instituted for all foods. Rather, it applies only to the five grains, rice, and a food called dochen. Tosfot (Eiruvin 30a) explains that the special bracha of mezonot is warranted only by foods of extreme significance: i.e. foods which people generally use as a meal staple (kviat seuda). As such, only the five grains and rice are worthy of borei minei mezonot, although - definitionally - all foods are referred to as mezonot.
However, our Yerushalmi does have halachic significance even in the realm of brachot. What if one erred and made the mezonot blessing upon a piece of chicken, a banana, or the like? Is his bracha valid b'dieved (post facto), or must he make a new, proper blessing for these foods?
Chayei Adam (58, see Nishmat Adam ad loc) rules that the borei minei mezonot is valid b'dieved, based upon the Yerushalmi. After all, even chicken and a banana are bona fide forms of mezonot. All foods are called mazon. True, they aren't eaten as staple fillers and are thus not awarded the singular status of borei minei mezonot on a l'chatchila (ideal) level. However, post facto the bracha did have meaning and relevance - declaring chicken as a type of mezonot is by no means a falsehood. As such, the mezonot blessing, b'dieved, allows him to eat his chicken.]
This ruling of the Chayei Adam is quoted by the Biur Halachah (167:10) as well.
[Note: There are other approaches to the bracha of mezonot as well. One such approach stresses the ability of the five grains and rice to satiate - as opposed to the norm of people to use them as a meal staple - as the factor responsible for their earning borei minei mezonot. This discussion is very much rooted in the Gemara Brachot 44a and Eiruvin 30a. We have adapted Chayei Adam's approach due to its relevance to our parsha.]
II
In either case, the five grains are deserving of borei minei mezonot due to the role they play in people's meals [or their satiating capacity].
Aside for bracha rishona though, the five grains are also marked by their own unique bracha achrona: al hamichya. However, al hamichya differs from mezonot in that rice gets a borei minei mezonot, yet receives no al hamichya. Why the difference? If rice is enough of a staple for a special bracha rishona, why should the bracha achrona be different?
One solution to this question is posited by the Rosh (Brachot 6:8). The Rosh maintains that the al hamichya blessing is not related to the staple/filling quality of the five grains. It is rather a product of the grains being part of the seven species of Eretz Yisrael. After all, the rest of the seven species also have their own bracha achrona, similar to al hamichya: i.e. al ha'eitz etc. The grains - wheat and barley - are also part of those seven species. The other three grains - usually translated as spelt, oats, and rye - are also sub-categories of wheat and barley (Tur O.C. 208). As such, the five grains - no different than dates - warrant a special bracha achrona. Rice, however, - as much as a staple as it may be - is not one of the seven species of Eretz Yisrael; accordingly, it is not worthy of a unique bracha achrona; only borei nefashot is recited.
Interested in another approach? See Tur O.C. 208 and Mishnah Brurah 208:29.