fix: detect jsx file mime type

[juliuss007]

Summary

• classify .jsx file attachments as JavaScript text instead of generic binary data
• add regression coverage for JSX MIME detection

Fixes PlatformNetwork/bounty-challenge#53029

Verification

• rustc +1.91.1 --test src/cortex-cli/src/run_cmd/mime.rs -o /tmp/cortex_mime_test_53029 && /tmp/cortex_mime_test_53029
• cargo +1.91.1 check -p cortex-cli --lib
• git diff --check HEAD^ HEAD
• cargo +1.91.1 fmt --package cortex-cli --check

Bounty claim: PlatformNetwork/bounty-challenge#53029