Add fallback for missing finfo_open

[MGatner]

Description
I've just encountered my second sizable hosting company that doesn't have the fileinfo extension available. This causes File->getMimeType() to fail:

CRITICAL - 2019-09-01 21:32:28 --> Call to undefined function CodeIgniter\Files\finfo_open()

Given that this is a "standard" setup it probably isn't safe to rely on this function. This PR checks for the function, and failing to find it reverts to the client's reported MIME type.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide