Fixes #872 Prevent fatal error if returned extension value is false

[Tabrisrp]

Description

Prevent a fatal error when the returned extension value is false. Previous version was only handling string values, but wp_check_filetype() can return false.

Fixes #872

Type of change

• Bug fix (non-breaking change which fixes an issue).

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote self-explanatory code about what it does.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.