docs: improve uploaded_files.rst

[kenjis]

Description

• add file upload sample code like CI3
• fix coding style of sample code
• add note about Validation rules
• decoration

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

I forgot to escape in the view files.

[kenjis]

Added note about Validation rules.