I think here you are breaking the previous logic or this else belong to if but having a wrong indentation. Seems there is a problem with loop indentation in emacs, because for me in this small example it also indents else on the same level as when:

CL-USER> (loop for i in (list NIL NIL 1 2 3 4 5)
               when i
                 if (< i 3)
                   collect (list :before-else i)
               else
                 collect (list :else i))
((:BEFORE-ELSE 1) (:BEFORE-ELSE 2) (:ELSE 3) (:ELSE 4) (:ELSE 5))

Let's rewrite this code to make it more readable?

Also, I don't like an explicit pathname-p argument. We already have a pathname in the options. So we can decide if we wanna to make a multipart request right in the make-request itself. I'd add a helper function (multipart-request-required-p options) which will return true if at least one of options is pathname. Also pathname-p argument to process-options should be renamed to return-alist-p - this way it will be more obvious that this function can return plist or alist depending on this mode.

Probably it will be better to use `(some #'pathnamep options) here, because we don't need a pathname as the result:

CL-USER> (some #'pathnamep (list 1 2 #P"foo" #P"bar" 5 6))
T
CL-USER> (find-if #'pathnamep (list 1 2 #P"foo" #P"bar" 5 6))
#P"foo"