Name plaintext export attachment files by plaintext hash so they can be located from the exported json.

app/src/main/java/org/thoughtcrime/securesms/backup/v2/local/LocalArchiver.kt

@@ -186,7 +186,7 @@ object LocalArchiver {
         val prefixDirs = HashMap<String, DocumentFile>()
         for (attachment in attachments) {
           if (cancellationSignal()) break
-          val mediaName = MediaName.forLocalBackupFilename(attachment.plaintextHash, attachment.localBackupKey.key)
+          val mediaName = MediaName.forPlaintextExportFilename(attachment.plaintextHash)
 
           try {
             val ext = attachment.contentType

core/models-jvm/src/main/java/org/signal/core/models/backup/MediaName.kt

@@ -20,6 +20,11 @@ value class MediaName(val name: String) {
     fun forThumbnailFromMediaName(mediaName: String) = MediaName("${mediaName}_thumbnail")
     fun forLocalBackupFilename(plaintextHash: ByteArray, localKey: ByteArray) = MediaName(Hex.toStringCondensed(CryptoUtil.sha256(plaintextHash + localKey)))
 
+    /**
+     * Since these are unencrypted, we don't want to create them via the local key.
+     */
+    fun forPlaintextExportFilename(plaintextHash: ByteArray) = MediaName(Hex.toStringCondensed(plaintextHash))
+
     /**
      * For java, since it struggles with value classes.
      */