Validate README format before displaying

Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/13543
GitLab version: 8.2.2

Expected behavior

If a file other than rdoc, txt, or other compatible formats is set as the README it shouldn't be used as the project's README file.

Observed Behavior

When pushing a Pages file to the project it tries to display it as seen on the attached ScreenShot.
pages_as_readme

Reference

This was reproduced here: https://gitlab.com/balameb/sample_app

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cc/ @JobV @vsizov

• gitlab-org/gitlab-ce#3499
• gitlab-org/gitlab-ce#1406

Milestone changed to 8.4

mentioned in issue #3499 (closed)

mentioned in issue #1406 (closed)

Thanks for the references @rspeicher and the milestone @JobV, I'll report back.

Reassigned to @dbalexandre

mentioned in merge request !2298 (merged)

Status changed to closed by commit d453b24a

mentioned in commit d453b24a