**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.
