base: support multiple MIME types per file extension

[connor4312]

Adds support for multiple MIME types to map to the same file extension by
allowing mapExtToMediaMimes values to be string or string[]. This enables
image/jpeg and image/jpg to both map to .jpg extension, fixing MCP image
responses that provide RFC-compliant image/jpeg MIME type.

• Changed MapExtToMediaMimes interface to support string | string[] values
• Updated getMediaMime() to return first MIME type from array if present
• Updated getExtensionForMimeType() to check both singular and array values
• Added .jpe, .jpeg, and .jpg to support both image/jpg and image/jpeg
• Added tests for getExtensionForMimeType with multiple MIME types

Fixes #291962

(Commit message generated by Copilot)

[Copilot]

Pull request overview

This pull request adds support for multiple MIME types to map to the same file extension, solving an issue where MCP image responses with the RFC-compliant image/jpeg MIME type were being displayed as "file.bin" instead of a proper image file.

Changes:

• Extended MapExtToMediaMimes interface to support string | string[] values, allowing multiple MIME types per extension
• Updated .jpe, .jpeg, and .jpg extensions to support both image/jpg and image/jpeg MIME types
• Modified getMediaMime() to return the first MIME type when an array is present
• Enhanced getExtensionForMimeType() to search within arrays of MIME types
• Added comprehensive test coverage for the new functionality

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/vs/base/common/mime.ts	Modified interface and functions to support multiple MIME types per extension; updated JPEG-related extensions to map to both image/jpg and image/jpeg
src/vs/base/test/common/mime.test.ts	Added test cases for getExtensionForMimeType with multiple MIME types including JPEG, audio/mpeg, and video/mp4

Comments suppressed due to low confidence (1)

src/vs/base/test/common/mime.test.ts:33

• The test suite doesn't include test cases for getMediaMime() with the new array functionality. Consider adding tests to verify that when getMediaMime() is called on a file with an extension that has multiple MIME types (e.g., '.jpg'), it returns the first MIME type from the array (e.g., 'image/jpg' based on current implementation).

Example test case:

test('getMediaMime with multiple MIME types', () => {
    assert.strictEqual(getMediaMime('file.jpg'), 'image/jpg');
    assert.strictEqual(getMediaMime('file.jpeg'), 'image/jpg');
});

This would ensure the function behaves as expected when encountering the new array values in mapExtToMediaMimes.

	test('getExtensionForMimeType', () => {
		// Note: for MIME types with multiple extensions (e.g., image/jpg -> .jpe, .jpeg, .jpg),
		// the function returns the first matching extension in iteration order
		assert.ok(['.jpe', '.jpeg', '.jpg'].includes(getExtensionForMimeType('image/jpg')!));
		// image/jpeg is an alias for image/jpg and should also return a valid extension
		assert.ok(['.jpe', '.jpeg', '.jpg'].includes(getExtensionForMimeType('image/jpeg')!));
		assert.strictEqual(getExtensionForMimeType('image/png'), '.png');
		assert.strictEqual(getExtensionForMimeType('image/gif'), '.gif');
		assert.strictEqual(getExtensionForMimeType('image/webp'), '.webp');
		assert.ok(['.mp2', '.mp2a', '.mp3', '.mpga', '.m2a', '.m3a'].includes(getExtensionForMimeType('audio/mpeg')!));
		assert.ok(['.mp4', '.mp4v', '.mpg4'].includes(getExtensionForMimeType('video/mp4')!));
		assert.strictEqual(getExtensionForMimeType('unknown/type'), undefined);
	});

[Copilot]

The comment states "the function returns the first matching extension in iteration order" but this is not quite accurate. For MIME types that appear in multiple extensions (like 'image/jpeg' which appears in '.jpe', '.jpeg', and '.jpg'), the function returns the extension that is checked first in the for...in loop, which depends on the order the properties were defined in the object. This is deterministic but may not be the "first" extension in the array.

Consider clarifying the comment to explain that for MIME types appearing in multiple extensions, any valid extension may be returned depending on object property iteration order, or consider making the behavior more explicit by documenting which extension will be preferred.

Suggested change

	// the function returns the first matching extension in iteration order
	assert.ok(['.jpe', '.jpeg', '.jpg'].includes(getExtensionForMimeType('image/jpg')!));
	// image/jpeg is an alias for image/jpg and should also return a valid extension
	// any of the associated extensions may be returned, depending on the underlying
	// object property iteration order used by getExtensionForMimeType.
	assert.ok(['.jpe', '.jpeg', '.jpg'].includes(getExtensionForMimeType('image/jpg')!));
	// image/jpeg is an alias for image/jpg and should also return one of these extensions

[Copilot]

The MIME types are ordered as ['image/jpg', 'image/jpeg'], but according to RFC 2046, image/jpeg is the official/standard MIME type while image/jpg is a non-standard variant. Consider reordering to ['image/jpeg', 'image/jpg'] to prefer the RFC-compliant MIME type when calling getMediaMime(), which returns the first element of the array.

This would ensure that when getting the MIME type for a .jpg file, the function returns the standard image/jpeg rather than the non-standard image/jpg.

Suggested change

	'.jpe': ['image/jpg', 'image/jpeg'],
	'.jpeg': ['image/jpg', 'image/jpeg'],
	'.jpg': ['image/jpg', 'image/jpeg'],
	'.jpe': ['image/jpeg', 'image/jpg'],
	'.jpeg': ['image/jpeg', 'image/jpg'],
	'.jpg': ['image/jpeg', 'image/jpg'],