To uphold means, among other things, “to maintain, to preserve intact,” and it was but a slight extension of this to arrive at “to keep in repair.”
It was in this latter sense that the word upholder was coined, to apply to one who dealt in the sale, manufacture, or repair of clothing and furniture.
Such a person was also called an upholdster (which has the equal meaning of “one who upholds”), and, by elision of the hardly pronounceable d, this became an upholster.
Hence the materials with which an upholster dealt were named upholstery, but by now with complete loss of any reference to clothing except occasionally in a figurative sense.
Also, in the course of time, the noun upholster became obsolete, to be replaced by the longer upholsterer.