Attempt to guess the content type of a file when opening with --raw (#13521)

# Description
Attempt to guess the content type of a file when opening with --raw and
set it in the pipeline metadata.

<img width="644" alt="Screenshot 2024-08-02 at 11 30 10"
src="https://github.com/user-attachments/assets/071f0967-c4dd-405a-b8c8-f7aa073efa98">


# User-Facing Changes
- Content of files can be directly piped into commands like `http post`
with the content type set appropriately when using `--raw`.

crates/nu-test-support/src/lib.rs

@@ -9,6 +9,7 @@ use std::process::ExitStatus;
 // Needs to be reexported for `nu!` macro
 pub use nu_path;
 
+#[derive(Debug)]
 pub struct Outcome {
     pub out: String,
     pub err: String,