[ticket/12949] Check if mime_content_type is actually callable

PHPBB3-12949
Merge pull request #2851 from marc1706/ticket/12949

[ticket/12949] Check if mime_content_type is actually callable
Merge branch 'develop-ascraeus' into develop

# By Marc Alexander
# Via Dhruv Goel (1) and Marc Alexander (1)
* develop-ascraeus:
  [ticket/12949] Check if mime_content_type is actually callable