When you are in the edit mode of a map project there is under the 'Page Content' aka 'Description' a big blue button named 'Mape File'. If you press this there opens a field where you may search your computer for a a .zip, .schematic, .mcworld, .mctemplate, .schem or .nbt Datafile (as these are the files that may contoin minecraft constructions). Now you just need to go to the folder where the file you want to upload is saved an select it. The only thing you need to take care for is, that the files needs to be smaller than 15mb; but honestly I've never has a problem with this max size for any single structure. Not in McEdit time of schematics or the modern nbt time. Anyways - now you just need to click on save and wait a few seconds and you are done. Now your upload is diretly saved on PMC and not on a second webside and the Previewer only works with creations saved on PMC itself. The alternate download is primary for large world saves which are mostly larger than 15kb today.