Improved rules of files

Done
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* **File**: `is_file()`;
* **Exists**: `file_exists()`.
* **Executable**: `is_executable()`;
* **Link**: `is_link()`;
* **Readable**: `is_readable()`;
* **Uploaded**: `is_uploaded_file()`;
* **Writeable**: `is_writeable()`.

Issue #50 will be disconsidered?