**Summary**
In preparation for fixing a cluster of bugs with automatic file-type
detection and paving the way for some reliability improvements, refactor
`unstructured.file_utils.filetype` module and improve thoroughness of
tests.
**Additional Context**
Factor type-recognition process into three distinct strategies that are
attempted in sequence. Attempted in order of preference,
type-recognition falls to the next strategy when the one before it is
not applicable or cannot determine the file-type. This provides a clear
basis for organizing the code and tests at the top level.
Consolidate the existing tests around these strategies, adding
additional cases to achieve better coverage.
Several bugs were uncovered in the process. Small ones were just fixed,
bigger ones will be remedied in following PRs.
