upload guide
document.getElementsByName("file_type")[0].value = 9;

centered text
function getTextWidth(text,font){var canvas=getTextWidth.canvas||(getTextWidth.canvas=document.createElement("canvas"));var context=canvas.getContext("2d");context.font=font;var metrics=context.measureText(text);return metrics.width}
var strr='';var strr_in=document.getElementById('showcase_8_notes').value.split('\n');document.getElementById('showcase_8_notes').value='';strr_in.forEach(function(sttr_this){var cleanString=sttr_this.replace(/\[(.*?)\]/g,"");cleanString=cleanString.replace(/\:(.*?)\:/g,"");strr='';var str_width=getTextWidth(cleanString,"13px Arial");for(var i=0;i<parseInt(((600-str_width)/4)/4);i++)
strr+="⠀";strr+=sttr_this;document.getElementById('showcase_8_notes').value+=strr+'\n'})

upload gif workshop
document.getElementsByName("file_type")[0].value = 11;

upload cgif screenshots
document.getElementsByName("file_type")[0].value = 5;

Steam full size background, F12 in chrome,
document.getElementsByName("image_width")[0].value = 999999;document.getElementsByName("image_height")[0].value = 1;