Use check_path in the send_file[_with_mime] examples

[Ogeon]

I forgot to use the check_path function in the examples for send_file and send_file_with_mime, so those are now showcasing irresponsible behaviour. People may be copy-pasting that code in their own projects, and thereby also copy-paste unsafe path handling.

These examples should therefore be updated to check the input path for invalid components, with check_path, like in examples/handler_storage.rs.