in src/memory.rs [174:181]
fn upload_mut(&mut self, id: &MultipartId) -> Result<&mut PartStorage> {
let parts = id
.parse()
.ok()
.and_then(|x| self.uploads.get_mut(&x))
.ok_or_else(|| Error::UploadNotFound { id: id.into() })?;
Ok(parts)
}