NEVER go to an interview in jeans. Ever.
A nice skirt and shoes is no more difficult as far as getting dressed and it makes all the difference in the world.
We just had someone show up for an interview in jeans and a T-shirt and her interview lasted 5 minutes. Had she been hired, she would have been wearing medical scrubs so no one expected her to wear a business suit and high heels for the interview, but showing up dressed the way she did got her sent right out the door.
Remember what I said in my last answer to you.....
Going through a staffing agency can be beneficial because if you aren't a good fit for one employer, you might be perfect for another one. They will be summing up your over all employability and if they send you to an actual employer, they will advise you how to dress.
For now, your interview is with the staffing agency. There is no guarantee they will send you directly for any interviews with the actual employer.
You need to concern yourself with dressing appropriately for the staffing agency and making a good impression on them first.
Just my opinion based on my personal experience with HR and staffing agencies.
Good luck!