system/Validation/FileRules.php
public function mime_in(string $blank = null, string $params, array $data): bool
	{
		// Grab the file name off the top of the $params
		// after we split it.
		$params = explode(',', $params);
		$name   = array_shift($params);

		$file = $this->request->getFile($name);

		if (is_null($file))
		{
			return false;
		}

		return in_array($file->getMimeType(), $params);
	}
public function mime_in(string $blank = null, string $params, array $data): bool
	{
		// Grab the file name off the top of the $params
		// after we split it.
		$params = explode(',', $params);
		$name   = array_shift($params);

		$file = $this->request->getFile($name);

		if (is_null($file))
		{
			return false;
		}

		return in_array($file->getMimeType(), $params);
	}
Checks to see if an uploaded file's mime type matches one in the parameter.

@param string|null $blank
@param string      $params
@param array       $data

@return boolean