fix(cp): avoid guessed content type for multipart uploads

[overtrue]

Related issue

Closes #191

Background

Large local-to-remote rc cp uploads use multipart upload. For files such as a generated .txt file, the CLI guessed text/plain from the extension and forwarded that value into the multipart create request. Some RustFS multipart create paths reject that inferred content type, while small single-part uploads still work.

Solution

Skip automatically guessed content types for multipart-sized uploads. Explicit --content-type values are still honored, and small single-part uploads keep the existing MIME guessing behavior.

Tests

• cargo fmt --all --check
• cargo test -p rustfs-cli commands::cp::tests::test_select_upload_content_type --lib
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

[Copilot]

Pull request overview

This PR addresses failures in rc cp multipart uploads caused by forwarding an automatically guessed Content-Type (e.g., text/plain) into the multipart create request, which some RustFS multipart paths reject. The change keeps MIME guessing for small single-part uploads, while ensuring multipart-sized uploads only send an explicit --content-type when provided.

Changes:

• Move content-type selection to occur after file size is known, enabling size-based selection.
• Add select_upload_content_type helper to skip guessed MIME types for multipart-sized uploads while honoring explicit --content-type.
• Add unit tests covering the helper’s selection behavior.

Comments suppressed due to low confidence (2)

crates/cli/src/commands/cp.rs:351

• select_upload_content_type uses file_size > MULTIPART_THRESHOLD to decide when to skip guessed MIME types, but MULTIPART_THRESHOLD is documented as “files at least this size use multipart upload” and upload_file logs/chooses the multipart progress bar on file_size >= MULTIPART_THRESHOLD. This boundary mismatch means a file exactly equal to the threshold will still forward the guessed content type even though it’s treated as multipart elsewhere. Align the comparison (and ideally share the same predicate as the actual multipart decision) so the behavior is consistent at the threshold.

fn select_upload_content_type<'a>(
    explicit_type: Option<&'a str>,
    guessed_type: Option<&'a str>,
    file_size: u64,
) -> Option<&'a str> {
    if file_size > MULTIPART_THRESHOLD {
        explicit_type
    } else {
        explicit_type.or(guessed_type)
    }

crates/cli/src/commands/cp.rs:895

• The new unit tests cover < threshold and > threshold, but don’t assert behavior for file_size == MULTIPART_THRESHOLD, which is currently ambiguous/inconsistent with the constant’s doc comment and the progress-bar condition. Add a boundary test for the exact-threshold size once the intended semantics (>= vs >) are clarified so regressions at the cutoff are caught.

    #[test]
    fn test_select_upload_content_type_uses_guess_for_small_files() {
        let selected =
            select_upload_content_type(None, Some("text/plain"), MULTIPART_THRESHOLD - 1);

        assert_eq!(selected, Some("text/plain"));
    }

    #[test]
    fn test_select_upload_content_type_skips_guess_for_multipart_files() {
        let selected =
            select_upload_content_type(None, Some("text/plain"), MULTIPART_THRESHOLD + 1);

        assert_eq!(selected, None);
    }

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