Improve example for handling file uploads

[Firstyear]

This is about:

• [ ] A typo
• [ ] Innaccurate/misleading documentation (e.g. technically incorrect advice)
• [ ] Undocumented code
• [ ] Outdated documentation
• [ x ] Other

Problem

https://github.com/yewstack/yew/blob/master/examples/file_upload/src/main.rs

This example is very good to show how to handle file's in input forms and then preview them, but what it is missing is how to submit the files to a backend webserver after the user submits. It's not exactly clear how to get from this example to actually uploading the file into the server, so having a fully worked example that can submit here would be excellent to help users.

Questionaire (Optional)

• [ ] I'd like to write this documentation
• [ ] I'd like to write this documentation but I'm not sure what's needed
• [ ] I don't have time to add this right now, but maybe later
• [ x ] I don't have the subject matter expertise to write the documentation / example.

[Somil-Shukla]

Please assign this issue to me.