Enable files to be set as value in `FormData`

[kiwiyou]

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.

[nilslice]

This is a great idea. Is this something you would like to work on?

[kiwiyou]

I'd love to. But there seems to be some issues that should be resolved first:

• Should we replace current append and set to File-aware variant or create a new one? Replacing them will be a breaking change.
  

  workers-rs/worker/src/formdata.rs

  Lines 79 to 89 in 07bf913

  	/// Appends a new value onto an existing key inside a `FormData` object, or adds the key if it
  	/// does not already exist.
  	pub fn append(&mut self, name: &str, value: &str) -> Result<()> {
  	self.0.append_with_str(name, value).map_err(Error::from)
  	}
  	/// Sets a new value for an existing key inside a `FormData` object, or adds the key/value if it
  	/// does not already exist.
  	pub fn set(&mut self, name: &str, value: &str) -> Result<()> {
  	self.0.set_with_str(name, value).map_err(Error::from)
  	}

• In what way should we handle this From implementation? Specifically, can we return &dyn AsRef<&str> from File?
  

  workers-rs/worker/src/formdata.rs

  Lines 103 to 112 in 07bf913

  	impl From<HashMap<&dyn AsRef<&str>, &dyn AsRef<&str>>> for FormData {
  	fn from(m: HashMap<&dyn AsRef<&str>, &dyn AsRef<&str>>) -> Self {
  	let mut formdata = FormData::new();
  	for (k, v) in m {
  	// TODO: determine error case and consider how to handle
  	formdata.set(k.as_ref(), v.as_ref()).unwrap();
  	}
  	formdata
  	}
  	}

I just ran into this issue after creating most of my project and I've been in disbelief that this isn't available. Using a worker as a switchboard to send and receive files just makes sense to me; the functionality as a service is severely limited without this.

[kiwiyou]

@sfbr As a workaround, you can manually build a FormData body and set appropriate headers to send multipart data. crate multipart will do this for you.

@kiwiyou Are you suggesting to send the request using that crate or to create the byte array and use it with this API? Additionally, I don't see where I would send FormData even if I did create it.

[kiwiyou]

Sorry for late response. I meant creating a form data, encoding it into a byte array, and attaching it as a request body. FormData cannot be directly sent with current API as you said.