Example:

from apiflask.schemas import FileSchema

@app.get('/files/<filename>')
@app.output(FileSchema, content_type='image/png')
def get_file(filename):
    return ...

fixes #447

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.