Use `isinstance` and `is not`.
Flag instance variables and methods non-public.

Only the `filename` needs to be obviously public. The other variables
and methods are compliant with PEP8's non-public naming scheme.
Change to snake_case for methods.
Use `endswith` and list comprehension.

The script that interfaces with the module was not using list
comprehension and was not compliant with the PEP8 standard for checking
extensions.
Fix isinstance issue
Change variables to follow snake case.
Merge branch 'feature/pep8' into develop