Add missing data parameter for MimetypeHandler.set_thumbnail.
Add missing data parameter for MimetypeHandler.set_thumbnail.

The abstract `MimetypeHandler` class defines a `set_thumbnail` method
which is always expected to be overridden by subclasses. Unfortunately
the method signature didn't match, which caused static analysis tools to
get mad. This change adds the missing `data` argument.

Testing Done:
Saw SA error disappear after adding missing param.

Reviewed at https://reviews.reviewboard.org/r/12109/

reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard