Feat/file upload example - #299

Description

Add File Upload example page + tests. Adds the real world pattern of () through two tests.

Fixes #218

Type of Change

• [ X ] Test addition or update

Testing

• [ X ] I have added tests that prove my fix is effective or that my feature works
• [ X ] All existing tests pass
• [ X ] New and existing unit tests pass locally
• [ X ] I have tested this change manually

Note: Pull requests without tests will not be reviewed. Please ensure you've added appropriate test coverage.

Test Details

Describe the tests you've added or updated:

• One test that uploads a valid image file and display the selected filename.
• One test that uploads a txt file and displays a visible error message.

Documentation

• [ X ] I have updated the documentation accordingly
• [ X ] I have added/updated code comments where necessary
• [ X ] The changes are reflected in the relevant documentation files

Screenshots (if applicable)