add an upload type with size constraints and acceptable file types

server/src/types/upload.ts

@@ -0,0 +1,5 @@
+// Shared constraints for user supplied image uploads
+export const ImageMimeTypes = ["image/jpeg", "image/png", "image/jpg"] as const;
+export type ImageMimeType = (typeof ImageMimeTypes)[number];
+
+export const MAX_IMAGE_SIZE_BYTES = 3 * 1024 * 1024; // 3MB