How can I encode a file into a base 64?

You need to encode yourself.

Here is a sample function you can reference.

function base64(fileNm) {
	var startDate = new Date();
	$.writeln("Base64 encode start time= " + startDate);
	var theFile = new File(fileNm);
	theFile.encoding = "binary";
	theFile.open("r");
	var binStr = theFile.read();
	theFile.close();
	var flNm = "/tmpfile.txt";
	var tmpFile = new File(flNm);
	tmpFile.open("w");	
	szB64 = new Array;
	var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";	// Accepted b64 chars
	for (j=0;j<64;j++){
		szB64[j] = keyStr.charAt(j);
		}
	var ecdStr = "";
	var counter = 0;
	var binLen = binStr.length - (binStr.length % 3); 
	for (i=0;i<binLen;i+=3) {		// Get 3 chars starting at the begining of the binary string
		ecdStr += szB64[binStr.charCodeAt (i) >> 2];
		ecdStr += szB64[((binStr.charCodeAt (i) & 0x3) << 4) | (binStr.charCodeAt (i+1) >> 4)];
		ecdStr += szB64[((binStr.charCodeAt (i+1) & 15) << 2) | (binStr.charCodeAt (i+2) >> 6)];
		ecdStr += szB64[binStr.charCodeAt (i+2) & 0x3f];
		counter +=4;
		if(counter > 75) {		// when hit the limit, write the line and reset counter
			tmpFile.writeln (ecdStr);
			ecdStr = "";
			counter = 0;
			}
		}

	switch (binStr.length % 3){
		case 2: //padding 1 characters.
			ecdStr += szB64[binStr.charCodeAt (i) >> 2];
			ecdStr += szB64[((binStr.charCodeAt (i) & 0x3) << 4) | (binStr.charCodeAt (i+1) >> 4)];
			ecdStr += szB64[((binStr.charCodeAt (i+1) & 15) << 2) | (binStr.charCodeAt (i+2) >> 6)];
			ecdStr += "=";
			tmpFile.writeln (ecdStr);	
		break;
		case 1: //padding 2 character.
			ecdStr += szB64[binStr.charCodeAt (i) >> 2];
			ecdStr += szB64[((binStr.charCodeAt (i) & 0x3) << 4) | (binStr.charCodeAt (i+1) >> 4)];
			ecdStr += "=="
			tmpFile.writeln (ecdStr);	
		break;
		default:
			tmpFile.writeln (ecdStr);	
		break;
		}
	tmpFile.close();
	var endDate = new Date();
	$.writeln("Base64 encode finish time = " + endDate);
	}