**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.
**Summary**
Replace conditional explicit import of partitioner modules in
`.partition.auto` with the new `_PartitionerLoader` class. This avoids
unbound variable warnings and is much less noisy.
`_PartitionerLoader` makes use of the new `FileType` property
`.importable_package_dependencies` to determine whether all required
packages are importable before dispatching the file to its partitioner.
It uses `FileType.extra_name` to form a helpful error message when a
dependency is not installed, so the caller knows which `pip install`
extra to specify to remedy the error.
`PartitionerLoader` uses the `FileType` properties
`.partitioner_module_qname` and `partitioner_function_name` to load
the partitioner once its dependencies are verified. Loaded partitioners
are cached with module lifetime scope for efficiency.
**Summary**
Elaborate the `FileType` enum to be a complete descriptor of file-types.
Add methods to allow `STR_TO_FILETYPE`, `EXT_TO_FILETYPE` and
`FILETYPE_TO_MIMETYPE` mappings to be replaced, removing those redundant
and noisy declarations.
In the process, fix some lingering file-type identification and
`.metadata.filetype` errors that had been skipped in the tests.
**Additional Context**
Gathering the various attributes of a file-type into the `FileType` enum
eliminates the duplication inherent in the separate `STR_TO_FILETYPE`
etc. mappings and makes access to those values convenient for callers.
These attributes include what MIME-type a file-type should record in
metadata and what MIME-types and extensions map to that file-type. These
values and others are made available as methods and properties directly
on the `FileType` class and members. Because all attributes are defined
in the `FileType` enum there is no risk of inconsistency across multiple
locations and any changes happen in one and only one place. Further
attributes and methods will be added in later commits to support other
file-type related operations like mapping to a partitioner and verifying
its dependencies are installed.
