upload.js

$(function(){
  $("#files").bind("change", handleFileSelection);
	var dropzone = $("#dropzone");
	dropzone.on("dragover",allowDrop);
	dropzone.on("drop",drop);
});

function handleFileSelection(event){
	upload(event.target.files);
}

function allowDrop(event){
	event.preventDefault();
}

function drop(event){
	event.preventDefault();
	upload(event.originalEvent.dataTransfer.files);
}

function upload(files){
	
	for (var i = 0; i < files.length; i++) {
		var file = files[i];
		if(file.type != "image/jpeg"){
			console.log("Unsupported type of " + file.name + ": " + file.type);
			continue;
		}
		
		console.log("Uploading " + file.name);
		
		(function(leFile) {
			$.ajax({
				type: "GET",
				url: "/credentials",
				cache: false,
				success: function(credentials){
					post(credentials, leFile);
				},
				error: function(){
					console.error("Could not fetch credentials from server");
				}
			});
		})(file);
	}
	
}

function post(credentials, file){
	
	var formData = new FormData();
	formData.append("key", credentials.key);
	formData.append("acl", credentials.acl);
	formData.append("Content-Type", credentials.content_type);
	formData.append("AWSAccessKeyId", credentials.access_key);
	formData.append("policy", credentials.policy)
	formData.append("signature", credentials.signature);
	formData.append("file", file);
	
	$.ajax({
		type: "POST",
		url: "https://" + credentials.bucket + ".s3.amazonaws.com",
		data: formData,
		processData: false, 
		contentType: false,
		cache: false,
		success: function(){
			console.log(file.name + " successfully uploaded");
		},
		error: function(){
			console.error("Could not upload " + file.name);
		}
	});
}

Thank you pangratz! Closures FTW!