add more notes to protobuf

proto/mf.proto

@@ -9,8 +9,11 @@ message MFFile {
   }
   Version version = 1;
   bytes innerMessage = 2;
+
+  // these are used solely to detect corruption/truncation
+  // and not for cryptographic integrity.
   uint64 size = 3;
-  bytes crc32 = 4;
+  bytes sha256 = 4;
 
   // think we might use gosignify instead of gpg:
   // github.com/frankbraun/gosignify
@@ -25,7 +28,10 @@ message MFFile {
 
 message MFFilePath {
     string path = 1;
-    // FIXME checksum/hash here
+    string ipfsHash = 2; // this is the DAG version that you can `ipfs get`
+    // more info: https://ethereum.stackexchange.com/questions/44506/ipfs-hash-algorithm/53233
+    bytes multiHash = 3; //this must be sha256 for now
+    optional string mimeType = 4;
 }
 
 message MFFileInner {