In a system such as

(defsystem :foo
  :component ((:file "foo")))

where foo.lsp exists and is recognized by the lisp implementation as a valid source file (ECL), ASDF imposes the file extension "lisp". Furthermore, adding our own extension

(defsystem :foo
  :component ((:file "foo.lsp")))

does not work and one has to resort to an EQL-specialized method (ugly!)

I have submitted a small patch that adds a new option :type that can be used with components of the type source-file