Fix mimeTypes TypeScript type (#464)

Author: charsleysa

core.d.ts

@@ -353,7 +353,7 @@ declare namespace core {
 	/**
 	Supported MIME types.
 	*/
-	const mimeTypes: readonly core.MimeType[];
+	const mimeTypes: Set<core.MimeType>;
 
 	/**
 	Detect the file type of a readable stream.

index.test-d.ts

@@ -41,7 +41,7 @@ expectType<Promise<FileTypeResult | undefined>>(FileType.fromBuffer(new ArrayBuf
 
 expectType<Set<FileType.FileExtension>>(FileType.extensions);
 
-expectType<readonly FileType.MimeType[]>(FileType.mimeTypes);
+expectType<Set<FileType.MimeType>>(FileType.mimeTypes);
 
 const readableStream = fs.createReadStream('file.png');
 const streamWithFileType = FileType.stream(readableStream);