Add file validators for File field

[uncle-lv]

fixes #253

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.

[uncle-lv]

I'm not sure if *Version changed* or *Version added* is 2.0.3(the next version) or 2.1.0(the milestones in #253).

[greyli]

Merged, thank you for this awesome feature!

I'm not sure if *Version changed* or *Version added* is 2.0.3(the next version) or 2.1.0(the milestones in #253).

I have changed it to 2.1.0.