1.
choose context variable (eg ctx.vars.file) to save your file from form
2-3 step:
See any file res xml to know what to save
ctx.vars.file -->has e.g @md5, @originalName.
var fileResXml = <fileRes contentType={contentType}
internalName={intName}
label={originalName} md5={md5Var}
name={destFileName} originalName={originalName}
xtkschema="xtk:fileRes"><folder id={folderId}/></fileRes>;
var fileResObj = xtk.fileRes.create(fileResXml);
fileResobj.PublishIfNeeded(); fileResobj.save();
var fileResId = *query resource id by unique internal name*;
fileResobj.save();
fileRfileResobj.save();esobj.save();