Undefined name: files --> files_inline

[cclauss]

files is an undefined name in this context which has the potential to raise NameError at runtime. This PR proposes to use __files_inline__ instead.

flake8 testing of https://github.com/allo-/firefox-profilemaker on Python 3.7.1

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./forms.py:67:25: F821 undefined name 'files'
                        files.update(option['files_inline'][choice])
                        ^
1     F821 undefined name 'files'
1

[allo-]

Thank you.