Fixed 'accept' attribute documentation

Added description about valid 'accept' parameter values: don't mix file types and mime types in an accept attribute value!

Current examples (upload images / pdfs) don't work due to mixed mime type / file extension definition in `accept` attribute.

See spec definition here: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input#attr-accept

*It's not literally stating that mixing values for `accept` is a wrong thing to do but tests using this directive prove that it only works when you either list mime types or file extensions. Not both.*