fix(shared): also encode file names that already look like an encoded-word

A literal ASCII file name such as "=?UTF-8?B?SGVsbG8=?=" was stored verbatim
but unwrapped to "Hello" when read back from S3 metadata. Encode such values
too so decodeS3MetadataValue can never misinterpret a literal name.

Author: ryanchou1994

packages/shared/s3MetadataEncoding.test.ts

@@ -33,6 +33,14 @@ describe("S3 metadata value encoding", () => {
     }
   });
 
+  it("round-trips a literal value that looks like an encoded word", () => {
+    const literal = "=?UTF-8?B?SGVsbG8=?=";
+    const encoded = encodeS3MetadataValue(literal);
+    expect(encoded).not.toBe(literal);
+    expect(encoded).toMatch(/^[\x20-\x7e]*$/);
+    expect(decodeS3MetadataValue(encoded)).toBe(literal);
+  });
+
   it("passes through values that are not encoded words", () => {
     expect(decodeS3MetadataValue("photo.jpg")).toBe("photo.jpg");
     expect(decodeS3MetadataValue("=?not-an-encoded-word")).toBe(

packages/shared/s3MetadataEncoding.ts

@@ -9,7 +9,9 @@ const ASCII_ONLY = /^[\x20-\x7e]*$/;
 const ENCODED_WORD = /^=\?UTF-8\?B\?([A-Za-z0-9+/=]*)\?=$/;
 
 export function encodeS3MetadataValue(value: string): string {
-  if (ASCII_ONLY.test(value)) {
+  // A literal value that already looks like an encoded-word must be encoded
+  // too, otherwise decodeS3MetadataValue would unwrap it on the way back.
+  if (ASCII_ONLY.test(value) && !ENCODED_WORD.test(value)) {
     return value;
   }
   return `=?UTF-8?B?${Buffer.from(value, "utf8").toString("base64")}?=`;