From page 240 of the Player's Handbook, find steed says, in part:
You summon a spirit that assumes the form of an unusually intelligent, strong and loyal steed, creating a long lasting bond with it. Appearing in an unoccupied space within range, the steed takes on the form that you choose, such as warhorse, a pony, a camel, an elk, or a mastiff....
When the steed drops to 0 hit points, it disappears, leaving behind no physical form. You can dismiss your steed at any time as an action, causing it to disappear. In either case, casting this spell again summons the same steed, restored to its hit point maximum....
It seems convenient for it to show up with the regular attire of a mount, and to have that stick with it. However, this basically gives it the functionality of the "leomund's secret chest" (p254) which is a level 4 spell with a 5,050gp cost.
What's to stop a player from loading the summoned warhorse with luggage and dismissing it until they feel like accessing it again?
I am not looking for "it's up to the DM" type answers. Rules as intended are best, rules as written will substitute, and info from official WotC sources is great too.