Add EntityInterface to FileEntity about file_entity

[arildm]

Hm, yes, I just added the EntityInterface typehint to the method in fe3ebb8, making the test pass. FileEntity implements that interface but not directly.
FileEntity ⇾ File ⇾ FileInterface ⇾ ContentEntityInterface ⇾ FieldableEntityInterface ⇾ EntityInterface

[LKS90]

Then you should get an exception about FileEntity not implementing the EntityInterface in the tests.

[LKS90]

No errors about that on the travis-ci build, nevermind then:

https://travis-ci.org/md-systems/file_entity/jobs/67208511

[Berdir]

The fix looks fine. FileEntity can not not implement that interface, it extends from a class that does. It's more likely that the parent method there was changed and that broke it. That also means that the related core patches for bringing this into core need to be updated if they haven't been already.