Intelligently set multipart file content-type

[ryanfowler]

No description provided.

[Copilot]

Pull request overview

This PR enhances multipart file upload functionality by intelligently detecting and setting content-type headers for files. Instead of using a generic content-type, the implementation now attempts to determine the correct MIME type by checking file extensions first, then falling back to content sniffing for extensionless files.

Changes:

• Added intelligent content-type detection for multipart file uploads via extension mapping and content sniffing
• Refactored file upload logic into a dedicated writeFilePart function
• Added comprehensive test coverage for the new content-type detection behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/multipart/multipart.go	Implements content-type detection via extension mapping and content sniffing, refactors file writing into writeFilePart function
internal/multipart/multipart_test.go	Adds unit tests for content-type detection with JSON files and extensionless JPEG files
integration/integration_test.go	Updates integration test to verify JPEG content-type detection using JPEG signature bytes

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