Remove the need for a directory for db export

Plus on import use the context directory.  We can actually write there
just fine.

src/blob.rs

@@ -242,9 +242,7 @@ impl<'a> BlobObject<'a> {
     /// including the dot.  E.g. "foo.txt" is returned as `("foo",
     /// ".txt")` while "bar" is returned as `("bar", "")`.
     ///
-    /// The extension part will always be lowercased.  Note that [`crate::imex::get_backup`]
-    /// relies on this for safety, if uppercase extensions are ever allowed it needs to be
-    /// adapted.
+    /// The extension part will always be lowercased.
     fn sanitise_name(name: &str) -> (String, String) {
         let mut name = name.to_string();
         for part in name.rsplit('/') {