Introduce a BlobObject type for blobs

This creates a specific type for blobs, with well defined conversions
at the borders.  It also introduces a strong type for the Param::File
value since that param is often used used by the public API to set
filenames using absolute paths, but then core changes the param to a
blob before it gets to the database.

This eliminates a few more functions with very mallable C-like
arguments behaviour which combine a number of operations in one.
Because blob filenames are stored so often in arbitrary strings this
does add more code when receiving those, until the storage is fixed.

File name sanitisation is now deletated to the sanitize-filename crate
which should do a slightly better job at this.

src/param.rs

@@ -1,9 +1,12 @@
 use std::collections::BTreeMap;
 use std::fmt;
+use std::path::PathBuf;
 use std::str;
 
 use num_traits::FromPrimitive;
 
+use crate::blob::{BlobError, BlobObject};
+use crate::context::Context;
 use crate::dc_mimeparser::SystemMessage;
 use crate::error;
 
@@ -204,10 +207,40 @@ impl Params {
     }
 }
 
+/// The value contained in [Param::File].
+///
+/// Because the only way to construct this object is from a valid
+/// UTF-8 string it is always safe to convert the value contained
+/// within the [ParamsFile::FsPath] back to a [String] or [&str].
+/// Despite the type itself does not guarantee this.
+#[derive(Debug)]
+pub enum ParamsFile<'c> {
+    FsPath(PathBuf),
+    Blob(BlobObject<'c>),
+}
+
+impl<'c> ParamsFile<'c> {
+    /// Parse the [Param::File] value into an object.
+    ///
+    /// If the value was stored into the [Params] correctly this
+    /// should not fail.
+    pub fn from_param(context: &'c Context, src: &str) -> Result<ParamsFile<'c>, BlobError> {
+        let param = match src.starts_with("$BLOBDIR/") {
+            true => ParamsFile::Blob(BlobObject::from_name(context, src.to_string())?),
+            false => ParamsFile::FsPath(PathBuf::from(src)),
+        };
+        Ok(param)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    use std::path::Path;
+
+    use crate::test_utils::*;
+
     #[test]
     fn test_dc_param() {
         let mut p1: Params = "\r\n\r\na=1\nf=2\n\nc = 3 ".parse().unwrap();
@@ -251,4 +284,24 @@ mod tests {
             .unwrap();
         assert_eq!(p1.get(Param::Forwarded).unwrap(), "cli%40deltachat.de");
     }
+
+    #[test]
+    fn test_params_file_fs_path() {
+        let t = dummy_context();
+        if let ParamsFile::FsPath(p) = ParamsFile::from_param(&t.ctx, "/foo/bar/baz").unwrap() {
+            assert_eq!(p, Path::new("/foo/bar/baz"));
+        } else {
+            assert!(false, "Wrong enum variant");
+        }
+    }
+
+    #[test]
+    fn test_params_file_blob() {
+        let t = dummy_context();
+        if let ParamsFile::Blob(b) = ParamsFile::from_param(&t.ctx, "$BLOBDIR/foo").unwrap() {
+            assert_eq!(b.as_name(), "$BLOBDIR/foo");
+        } else {
+            assert!(false, "Wrong enum variant");
+        }
+    }
 }