Re-work some in-creation file handling

This effectively reverts
https://github.com/deltachat/deltachat-core-rust/pull/964 for chat.rs,
which in that PR was thought to fix something.  So maybe something is
still broken?  But after improving tests the previous code seems to be
correct.

- Update Python bindings to not always use dc_prepare_msg path when
  sending messages with attachements.  When using dc_prepare_msg the
  blobs need to be created in the blobdir since they will not get
  copied and many tests where not doing this.

- Add a test that ensures that calling dc_prepare_msg with a
  file **not** in the blobdir fails.

- Add a test that ensures that calling dc_send_msg directly with a
  file **not** in the blobdir copies the file to the blobdir.  This
  test cheats a little by knowing what the filename in the blobdir
  will be which is implementation-dependent and thus a bit brittle.
  But for now it proves correct behaviour so let's go with this.

- Improve the test_forward_increation test to ensure that the
  in-creation file only has it's final state before calling
  dc_send_msg.  This checks the correct file data is sent out and not
  the preparing data, this fails with the chat.rs changes in
  #964 (reverted here to make this work again).  Also fix the test to
  actually create the in-creation file in the blobdir.

- Fix test_send_file_twice_unicode_filename_mangling to not use
  in-creation.  It was not creating it's files in the blobdir and that
  is an error when using in-creation and it didn't seem it was trying
  to test something about the in-creation logic (which is tested in
  test_increation.py already).

- Fix Message._msgtate code which presumably was not used before?

- Rename `BlobObject::create_from_path` to
  `BlobObject::new_from_path`.  All the `BlobObject::create*` calls
  now always create new files which is much more consistent.  APIs
  should do what is obious.

src/blob.rs

@@ -159,7 +159,7 @@ impl<'a> BlobObject<'a> {
     /// This merely delegates to the [BlobObject::create_and_copy] and
     /// the [BlobObject::from_path] methods.  See those for possible
     /// errors.
-    pub fn create_from_path(
+    pub fn new_from_path(
         context: &Context,
         src: impl AsRef<Path>,
     ) -> std::result::Result<BlobObject, BlobError> {
@@ -559,14 +559,14 @@ mod tests {
 
         let src_ext = t.dir.path().join("external");
         fs::write(&src_ext, b"boo").unwrap();
-        let blob = BlobObject::create_from_path(&t.ctx, &src_ext).unwrap();
+        let blob = BlobObject::new_from_path(&t.ctx, &src_ext).unwrap();
         assert_eq!(blob.as_name(), "$BLOBDIR/external");
         let data = fs::read(blob.to_abs_path()).unwrap();
         assert_eq!(data, b"boo");
 
         let src_int = t.ctx.get_blobdir().join("internal");
         fs::write(&src_int, b"boo").unwrap();
-        let blob = BlobObject::create_from_path(&t.ctx, &src_int).unwrap();
+        let blob = BlobObject::new_from_path(&t.ctx, &src_int).unwrap();
         assert_eq!(blob.as_name(), "$BLOBDIR/internal");
         let data = fs::read(blob.to_abs_path()).unwrap();
         assert_eq!(data, b"boo");
@@ -576,7 +576,7 @@ mod tests {
         let t = dummy_context();
         let src_ext = t.dir.path().join("autocrypt-setup-message-4137848473.html");
         fs::write(&src_ext, b"boo").unwrap();
-        let blob = BlobObject::create_from_path(&t.ctx, &src_ext).unwrap();
+        let blob = BlobObject::new_from_path(&t.ctx, &src_ext).unwrap();
         assert_eq!(
             blob.as_name(),
             "$BLOBDIR/autocrypt-setup-message-4137848473.html"

src/chat.rs

@@ -725,20 +725,11 @@ fn prepare_msg_blob(context: &Context, msg: &mut Message) -> Result<(), Error> {
     if msg.type_0 == Viewtype::Text {
         // the caller should check if the message text is empty
     } else if msgtype_has_file(msg.type_0) {
-        let blob = if let Some(f) = msg.param.get_file(Param::File, context)? {
-            match f {
-                ParamsFile::Blob(blob) => blob,
-                ParamsFile::FsPath(path) => {
-                    // path is outside the blobdir, let's copy
-                    let blob = BlobObject::create_and_copy(context, path)?;
-                    msg.param.set(Param::File, blob.as_name());
-
-                    blob
-                }
-            }
-        } else {
-            bail!("Attachment missing for message of type #{}", msg.type_0);
-        };
+        let blob = msg
+            .param
+            .get_blob(Param::File, context, !msg.is_increation())?
+            .ok_or_else(|| format_err!("Attachment missing for message of type #{}", msg.type_0))?;
+        msg.param.set(Param::File, blob.as_name());
 
         if msg.type_0 == Viewtype::File || msg.type_0 == Viewtype::Image {
             // Correct the type, take care not to correct already very special

src/config.rs

@@ -128,7 +128,7 @@ impl Context {
     pub fn set_config(&self, key: Config, value: Option<&str>) -> crate::sql::Result<()> {
         match key {
             Config::Selfavatar if value.is_some() => {
-                let blob = BlobObject::create_from_path(&self, value.unwrap())?;
+                let blob = BlobObject::new_from_path(&self, value.unwrap())?;
                 self.sql.set_raw_config(self, key, Some(blob.as_name()))
             }
             Config::InboxWatch => {

src/param.rs

@@ -250,7 +250,7 @@ impl Params {
         let file = ParamsFile::from_param(context, val)?;
         let blob = match file {
             ParamsFile::FsPath(path) => match create {
-                true => BlobObject::create_from_path(context, path)?,
+                true => BlobObject::new_from_path(context, path)?,
                 false => BlobObject::from_path(context, path)?,
             },
             ParamsFile::Blob(blob) => blob,