Конечный вариант:
<a href="#" id="dlLink" download="playlist.m3u">Download playlist</a>
<script>
/**
*
* Base64 encode / decode
* http://www.webtoolkit.info/
*
**/
var Base64={_keyStr:"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",encode:function(c){for(var a="",d,b,f,g,h,e,i=0,c=Base64._utf8_encode(c);i<c.length;)d=c.charCodeAt(i++),b=c.charCodeAt(i++),f=c.charCodeAt(i++),g=d>>2,d=(d&3)<<4|b>>4,h=(b&15)<<2|f>>6,e=f&63,isNaN(b)?h=e=64:isNaN(f)&&(e=64),a=a+this._keyStr.charAt(g)+this._keyStr.charAt(d)+this._keyStr.charAt(h)+this._keyStr.charAt(e);return a},decode:function(c){for(var a="",d,b,f,g,h,e=0,c=c.replace(/[^A-Za-z0-9\+\/\=]/g,"");e<c.length;)d=this._keyStr.indexOf(c.charAt(e++)),b=this._keyStr.indexOf(c.charAt(e++)),g=this._keyStr.indexOf(c.charAt(e++)),h=this._keyStr.indexOf(c.charAt(e++)),d=d<<2|b>>4,b=(b&15)<<4|g>>2,f=(g&3)<<6|h,a+=String.fromCharCode(d),64!=g&&(a+=String.fromCharCode(b)),64!=h&&(a+=String.fromCharCode(f));return a=Base64._utf8_decode(a)},_utf8_encode:function(c){for(var c=c.replace(/\r\n/g,"\n"),a="",d=0;d<c.length;d++){var b=c.charCodeAt(d);128>b?a+=String.fromCharCode(b):(127<b&&2048>b?a+=String.fromCharCode(b>>6|192):(a+=String.fromCharCode(b>>12|224),a+=String.fromCharCode(b>>6&63|128)),a+=String.fromCharCode(b&63|128))}return a},_utf8_decode:function(c){for(var a="",d=0,b=c1=c2=0;d<c.length;)b=c.charCodeAt(d),128>b?(a+=String.fromCharCode(b),d++):191<b&&224>b?(c2=c.charCodeAt(d+1),a+=String.fromCharCode((b&31)<<6|c2&63),d+=2):(c2=c.charCodeAt(d+1),c3=c.charCodeAt(d+2),a+=String.fromCharCode((b&15)<<12|(c2&63)<<6|c3&63),d+=3);return a}};
</script>
<script>
dlLink.href = "data:audio/x-mpegurl;base64," + Base64.encode("file content");
</script>