open: Assign content_type metadata for filetypes not handled with a from converter (#14670)

# Description

Filetypes which are converted during `open` should not have (and have
not had) a `content_type` metadata field. However, filetypes which
aren't converted now behave the same as with `--raw` and assign the
appropriate `content_type`.

## Before

```nushell
open toolkit.nu | metadata
# => ╭────────┬────────────────────────────────────────────╮
# => │ source │ /home/ntd/src/ntd-forks/nushell/toolkit.nu │
# => ╰────────┴────────────────────────────────────────────

open --raw toolkit.nu | metadata
# => ╭──────────────┬────────────────────────────────────────────╮
# => │ source       │ /home/ntd/src/ntd-forks/nushell/toolkit.nu │
# => │ content_type │ application/x-nuscript                     │
# => ╰──────────────┴────────────────────────────────────────────╯

open script.py | metadata
# => ╭────────┬─────────────────────────────╮
# => │ source │ /home/ntd/testing/script.py │
# => ╰────────┴─────────────────────────────╯

open Cargo.toml | metadata
# => ╭────────┬────────────────────────────────────────────╮
# => │ source │ /home/ntd/src/ntd-forks/nushell/Cargo.toml │
# => ╰────────┴────────────────────────────────────────────╯
```

## After

```nushell
# Not converted, so adds content_type
open toolkit.nu | metadata
# => ╭──────────────┬────────────────────────────────────────────╮
# => │ source       │ /home/ntd/src/ntd-forks/nushell/toolkit.nu │
# => │ content_type │ application/x-nuscript                     │
# => ╰──────────────┴────────────────────────────────────────────╯

# Not converted, so adds content_type
open --raw toolkit.nu | metadata
# => ╭──────────────┬────────────────────────────────────────────╮
# => │ source       │ /home/ntd/src/ntd-forks/nushell/toolkit.nu │
# => │ content_type │ application/x-nuscript                     │
# => ╰──────────────┴────────────────────────────────────────────╯

# Not converted, so adds content_type
open script.py | metadata
# => ╭──────────────┬─────────────────────────────╮
# => │ source       │ /home/ntd/testing/script.py │
# => │ content_type │ text/plain                  │
# => ╰──────────────┴─────────────────────────────

# Converted, so does not add content_type (no change)
open Cargo.toml | metadata
# => ╭────────┬────────────────────────────────────────────╮
# => │ source │ /home/ntd/src/ntd-forks/nushell/Cargo.toml │
# => ╰────────┴────────────────────────────────────────────╯
```

# User-Facing Changes

`open <file>` assigns the appropriate content type when the filetype is
not converted via a `from <format>`.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-command/tests/commands/open.rs

@@ -403,3 +403,48 @@ fn test_content_types_with_open_raw() {
         assert!(result.out.contains("application/yaml"));
     })
 }
+
+#[test]
+fn test_metadata_without_raw() {
+    Playground::setup("open_files_content_type_test", |dirs, _| {
+        let result = nu!(cwd: dirs.formats(), "(open random_numbers.csv | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open random_numbers.csv | metadata | get source?");
+        assert!(result.out.contains("random_numbers.csv"));
+        let result = nu!(cwd: dirs.formats(), "(open caco3_plastics.tsv | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open caco3_plastics.tsv | metadata | get source?");
+        assert!(result.out.contains("caco3_plastics.tsv"));
+        let result = nu!(cwd: dirs.formats(), "(open sample-simple.json | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open sample-simple.json | metadata | get source?");
+        assert!(result.out.contains("sample-simple.json"));
+        // Only when not using nu_plugin_formats
+        let result = nu!(cwd: dirs.formats(), "open sample.ini | metadata");
+        assert!(result.out.contains("text/plain"));
+        let result = nu!(cwd: dirs.formats(), "open sample.ini | metadata | get source?");
+        assert!(result.out.contains("sample.ini"));
+        let result = nu!(cwd: dirs.formats(), "(open sample_data.xlsx | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open sample_data.xlsx | metadata | get source?");
+        assert!(result.out.contains("sample_data.xlsx"));
+        let result = nu!(cwd: dirs.formats(), "open sample_def.nu | metadata | get content_type?");
+        assert_eq!(result.out, "application/x-nuscript");
+        let result = nu!(cwd: dirs.formats(), "open sample_def.nu | metadata | get source?");
+        assert!(result.out.contains("sample_def"));
+        // Only when not using nu_plugin_formats
+        let result = nu!(cwd: dirs.formats(), "open sample.eml | metadata | get content_type?");
+        assert_eq!(result.out, "message/rfc822");
+        let result = nu!(cwd: dirs.formats(), "open sample.eml | metadata | get source?");
+        assert!(result.out.contains("sample.eml"));
+        let result = nu!(cwd: dirs.formats(), "(open cargo_sample.toml | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open cargo_sample.toml | metadata | get source?");
+        assert!(result.out.contains("cargo_sample.toml"));
+        let result =
+            nu!(cwd: dirs.formats(), "(open appveyor.yml | metadata | get content_type?) == null");
+        assert_eq!(result.out, "true");
+        let result = nu!(cwd: dirs.formats(), "open appveyor.yml | metadata | get source?");
+        assert!(result.out.contains("appveyor.yml"));
+    })
+}