This is probably the most difficult part of the folio to produce, but it may be as important as any other part of your application since many places now explicitly ask for this in their job announcements. Instead of expanding your application letter, provide a separate sheet that sets this forth; include it in the portfolio, but also consider it as a stand-alone sheet to include in those applications where such a statement is asked for.
We all have some sort of teaching philosophy (even if it’s not yet been made explicit); we have answers to questions about what we expect of our students and how we think we should get it. But abstracting a reasonable statement of that "philosophy" from our classroom practice and experience is difficult, and it’s even more difficult to avoid doing it without producing platitudes ("collaborative learning," "clearly articulated expectations," "critical thinking," "active learning," "transparent grading criteria"…).
So, be concrete and exemplary. Focusing on a model course and/or model assignments will allow you to define the context of your teaching and the nature of your classroom activities. Use a few concrete examples to illustrate your ‘philosophy’ and give it substance and practical immediacy. This will show that you have not only taught, but have thought about teaching, and about becoming a better teacher.