fix rfc-2231 regex (multi digit continuation), add tests

imap-core/lib/indexer/parse-mime-tree.js

@@ -305,11 +305,11 @@ class MIMEParser {
             // which are encoded with lang/charset info as well as a continuation.
             // See https://tools.ietf.org/html/rfc2231 section 4.1.
 
-            if ((match = key.match(/^([^*]+)\*(\d)?\*?$/))) {
+            if ((match = key.match(/^([^*]+)\*(\d+)?\*?$/))) {
                 if (!processEncodedWords[match[1]]) {
                     processEncodedWords[match[1]] = [];
 
-                    // Additionally allow RFC5987 section 3.2.1 encoded values
+                    // Additionally allow RFC2231 encoded values
                     if (key.match(/^([^*]+)\*(?:\d+\*)?$/)) {
                         // must have charset
                         charsetRequired.add(processEncodedWords[match[1]]);

imap-core/test/parse-mime-tree-test.js

@@ -400,6 +400,24 @@ describe('#parseFilename (MIMEParser-driven)', function () {
         });
     });
 
+    it('continuation concatenates out-of-order sections in numeric order (unencoded)', function () {
+        let headers =
+            'Content-Disposition: attachment; ' +
+            'filename*0="part-0"; filename*3="part-3"; filename*2="part-2"; filename*5="part-5"; filename*4="part-4"';
+        expect(parseFilename(headers)).to.deep.equal({
+            filename: 'part-0part-2part-3part-4part-5',
+            source: 'filename*0'
+        });
+    });
+
+    it('continuation supports multi-digit section numbers (unencoded)', function () {
+        let headers = 'Content-Disposition: attachment; filename*0="part-A"; filename*10="part-K"';
+        expect(parseFilename(headers)).to.deep.equal({
+            filename: 'part-Apart-K',
+            source: 'filename*0'
+        });
+    });
+
     it('continuation with missing segment concatenates existing parts (encoded)', function () {
         let headers = "Content-Disposition: attachment; filename*0*=UTF-8''A; filename*2*=C";
         expect(parseFilename(headers)).to.deep.equal({
@@ -408,6 +426,14 @@ describe('#parseFilename (MIMEParser-driven)', function () {
         });
     });
 
+    it('continuation concatenates out-of-order sections in numeric order (encoded)', function () {
+        let headers = "Content-Disposition: attachment; filename*0*=UTF-8''A; filename*3*=D; filename*2*=C; filename*5*=F; filename*4*=E";
+        expect(parseFilename(headers)).to.deep.equal({
+            filename: 'ACDEF',
+            source: 'filename*0*'
+        });
+    });
+
     // 4) Malformed/no-charset behavior in current MIMEParser (lenient/quirky by design).
     it('malformed filename* with empty charset does not fall back automatically', function () {
         let headers = "Content-Disposition: attachment; filename*=''r%C3%A9sum%C3%A9.pdf";