TypeError on `Shape\Drawing\File::getMimeType()`

[nreynis]

I got the following stacktrace in a production environment:

TypeError: PhpOffice\PhpPresentation\Shape\Drawing\File::getMimeType(): Return value must be of type string, false returned
/vendor/phpoffice/phppresentation/src/PhpPresentation/Shape/Drawing/File.php:86 PhpOffice\PhpPresentation\Shape\Drawing\File::getMimeType
/vendor/phpoffice/phppresentation/src/PhpPresentation/Writer/PowerPoint2007/ContentTypes.php:134 PhpOffice\PhpPresentation\Writer\PowerPoint2007\ContentTypes::render
/vendor/phpoffice/phppresentation/src/PhpPresentation/Writer/PowerPoint2007.php:118 PhpOffice\PhpPresentation\Writer\PowerPoint2007::save

I didn't managed to reproduce the issue and I assume it might be a fringe issue with NFS synchronization.

However, this does not change the fact that mime_content_type might return false if anything goes wrong.
This code should either handle the error or turn it into a PhpPresentationException.
Having a TypeError is not desirable.