Use detected MIME types for OpenAI image attachments

[dhrupo]

Summary

Fix OpenAI image attachment mapping for local and stored image files.

Problem

Files\Image::fromPath() could produce an invalid OpenAI image data URL because the OpenAI attachment mapper used the raw image MIME property instead of the file's detected MIME type. When the MIME property was unset, requests could be sent as data:;base64,..., which OpenAI rejects with a 400 error.

Fixes #417

Solution

Updated the OpenAI attachment mapper to use mimeType() for local and stored image attachments, while preserving the existing image/png fallback when MIME detection is unavailable.

Tests

Added regression tests covering:

• local .jpg images mapped as data:image/jpeg;base64,...
• stored .jpg images mapped as data:image/jpeg;base64,...