filter_files() in class-base-check.php should be able to accept an array of file types.

[emrikol]

When working on GitHub issue #255, I have ran into trouble because filter_files() only accepts one file type at a time.

This would require extra for loops and iterations for each extra file type.

A solution I have is to modify filter_files() to alternatively accept an array() as its $type parameter, iterate over each type in the array, and return all of the files at once.

A pull request with a proposed fix should follow shortly.