system/Files/File.php
public function getMimeType(): string
	{
		$finfo    = finfo_open(FILEINFO_MIME_TYPE);
		$mimeType = finfo_file($finfo, $this->getRealPath());
		finfo_close($finfo);
		return $mimeType;
	}
public function getMimeType(): string
	{
		$finfo    = finfo_open(FILEINFO_MIME_TYPE);
		$mimeType = finfo_file($finfo, $this->getRealPath());
		finfo_close($finfo);
		return $mimeType;
	}
Retrieve the media type of the file. SHOULD not use information from
the $_FILES array, but should use other methods to more accurately
determine the type of file, like finfo, or mime_content_type().

@return string|null The media type we determined it to be.