Move file upload into SMTP send job.

- This adds params for the upload URL and local file path, so that the
actual upload can happen in the send job.
- This also moves the URL generation to the client side so that we can
  generate a valid URL before the upload (because the MIME rendering of
  the mail message happens earlier and we want to include the URL there)

src/upload.rs

@@ -1,11 +1,12 @@
 use crate::context::Context;
 use crate::error::{bail, Result};
 use async_std::path::PathBuf;
+use rand::Rng;
 
 /// Upload file to a HTTP upload endpoint.
-pub async fn upload_file(_context: &Context, endpoint: String, file: PathBuf) -> Result<String> {
+pub async fn upload_file(_context: &Context, url: String, filepath: PathBuf) -> Result<String> {
     // TODO: Use tokens for upload, encrypt file with PGP.
-    let response = surf::post(endpoint).body_file(file)?.await;
+    let response = surf::put(url).body_file(filepath)?.await;
     if let Err(err) = response {
         bail!("Upload failed: {}", err);
     }
@@ -15,3 +16,17 @@ pub async fn upload_file(_context: &Context, endpoint: String, file: PathBuf) ->
         Err(err) => bail!("Invalid response from upload: {}", err),
     }
 }
+
+/// Generate a random URL based on the provided endpoint.
+pub fn generate_upload_url(_context: &Context, endpoint: String) -> String {
+    const CROCKFORD_ALPHABET: &[u8] = b"0123456789abcdefghjkmnpqrstvwxyz";
+    const FILENAME_LEN: usize = 27;
+    let mut rng = rand::thread_rng();
+    let filename: String = (0..FILENAME_LEN)
+        .map(|_| {
+            let idx = rng.gen_range(0, CROCKFORD_ALPHABET.len());
+            CROCKFORD_ALPHABET[idx] as char
+        })
+        .collect();
+    format!("{}{}", endpoint, filename)
+}