Enable files to be set as value in `FormData`

It's quite strange to have `FormData::get` returning `FormEntry` with `FormData::set` and `FormData::append` accepting only `&str`. It would be great if we can insert `File` or something like it directly into `FormData`.

Start by tracing the FormData::get, FormData::set, and FormData::append entry points and how FormEntry represents values. Check how File-like values are exposed in the Workers API, then define the supported input and verify that files can be inserted and retrieved without breaking existing string behavior.

Written by the indexing model from the issue text.