clarified how/when to use client or guessed mime-types and extensions on a File instance (closes #5039)
[HttpFoundation] added some more phpdocs to clarify usage
[HttpFoundation] added a way to inject a custom magic file into FileinfoMimeTypeGuesser (closes #6963)
[HttpFoundation] fixed phpdoc
[HttpFoundation] updated the list of known mime types (sync from Apache)
merged branch fabpot/mimetypes (PR #7809)

This PR was merged into the master branch.

Discussion
----------

[WIP][HttpFoundation] File mime types and extensions

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5039, #6963, #5258, #7415, #5615
| License       | MIT
| Doc PR        | n/a

This pull request addresses most of the issues regarding mime types and extension on File instances.

I've also created an issue on the docs to add more documentation about how things work (see symfony/symfony-docs#2542).

Commits
-------

acc2b6a [HttpFoundation] updated the list of known mime types (sync from Apache)
f85cdd2 [HttpFoundation] fixed phpdoc
1aa68da [HttpFoundation] added a way to inject a custom magic file into FileinfoMimeTypeGuesser (closes #6963)
29b5413 [HttpFoundation] added some more phpdocs to clarify usage
ee784fb clarified how/when to use client or guessed mime-types and extensions on a File instance (closes #5039)