Why doesn't my picture caption show up when I send a picture upload to Facebook via MMS?

I’m sending my picture that I take with my phone to Facebook via the email address that I have. Usually I send it with the email option on my phone but now something is wrong with that so now I use MMS.

But everytime when I send a picture I put a caption on the subject bar thing on the MMS thing but when it loads on Facebook there’s no caption.

How can this be fixed? I want the caption to be shown but it’s not showing while I have put a caption on the subject thing.