[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
| License       | MIT
| Doc PR        | n/a

This pull request addresses most of the issues regarding mime types and extension on File instances.