A search on Linguee shows that, in most of the cases, no single word is used to translate "uploader".
If you are referring to a program that uploads files, expressions like utilidad/programa/herramienta de subida de archivos are common (or herramienta de upload, which is used in some Latin American countries but not in Spain).
Some programs do use the word cargador, but the DLE does not include this meaning of the word cargador.
If you are referring to a person that uploads files, I have not found any example of a single word used for this in Spanish. Expressions like la persona que ha subido el archivo are used often, or other terms that are only valid in some cases like autor, or publicador.