* Adding a check to see if the posted value's source path isn't null or empty.
* Moving validation logic to proper files
* Moved logic to a required validator
* Adding tests to ensure validation works
* Minor tidy up: XML header comments, re-use in tests, clarified test names.
* Adding unit tests for file upload validation
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Co-authored-by: Andy Butland <abutland73@gmail.com>
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