diff --git a/app/src/main/java/net/gsantner/markor/format/todotxt/TodoTxtActionButtons.java b/app/src/main/java/net/gsantner/markor/format/todotxt/TodoTxtActionButtons.java
index 4c51992032..b6651d8cf9 100644
--- a/app/src/main/java/net/gsantner/markor/format/todotxt/TodoTxtActionButtons.java
+++ b/app/src/main/java/net/gsantner/markor/format/todotxt/TodoTxtActionButtons.java
@@ -21,6 +21,7 @@
 
 import net.gsantner.markor.R;
 import net.gsantner.markor.format.ActionButtonBase;
+import net.gsantner.markor.format.FormatRegistry;
 import net.gsantner.markor.frontend.MarkorDialogFactory;
 import net.gsantner.markor.frontend.textview.TextViewUtils;
 import net.gsantner.markor.model.Document;
@@ -242,7 +243,7 @@ public void archiveDoneTasks() {
                 doneContents.append(TodoTxtTask.tasksToString(move)).append("\n");
 
                 // Write to done file
-                if (new Document(doneFile).saveContent(getActivity(), doneContents.toString())) {
+                if (new Document(doneFile, FormatRegistry.FORMAT_TODOTXT).saveContent(getActivity(), doneContents.toString())) {
                     final String tasksString = TodoTxtTask.tasksToString(keep);
                     _hlEditor.setText(tasksString);
                     TextViewUtils.setSelectionFromOffsets(_hlEditor, offsets);
@@ -514,4 +515,4 @@ public void onCreate(final Bundle savedInstanceState) {
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextSyntaxHighlighter.java b/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextSyntaxHighlighter.java
index c9d441815e..f0893d3d21 100644
--- a/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextSyntaxHighlighter.java
+++ b/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextSyntaxHighlighter.java
@@ -41,7 +41,6 @@ public class WikitextSyntaxHighlighter extends SyntaxHighlighterBase {
     public final static Pattern CHECKLIST_LEFT_ARROW = Pattern.compile("(?<=(\\n|^))\t*(\\[)(<)(])(?= )");
     public final static Pattern SUBSCRIPT = Pattern.compile("(_\\{(?!~)(.+?)\\})");
     public final static Pattern SUPERSCRIPT = Pattern.compile("(\\^\\{(?!~)(.+?)\\})");
-    public final static Pattern ZIMHEADER_CONTENT_TYPE_ONLY = Pattern.compile("^\\s*Content-Type:\\s*text/x-zim-wiki");
     public final static Pattern ZIMHEADER = Pattern.compile(
             "^Content-Type: text/x-zim-wiki(\r\n|\r|\n)" +
                     "Wiki-Format: zim \\d+\\.\\d+(\r\n|\r|\n)" +
diff --git a/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextTextConverter.java b/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextTextConverter.java
index 11ccb3a3e7..597f4cac48 100644
--- a/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextTextConverter.java
+++ b/app/src/main/java/net/gsantner/markor/format/wikitext/WikitextTextConverter.java
@@ -36,6 +36,8 @@
  */
 @SuppressWarnings("WeakerAccess")
 public class WikitextTextConverter extends TextConverterBase {
+    private static final Pattern ZIMHEADER_CONTENT_TYPE_ONLY = Pattern.compile("^\\s*Content-Type:\\s*text/x-zim-wiki");
+
     // Zim Source View plugin: {{{code: lang="..." ...\n...\n}}}
     private static final Pattern CODE_BLOCK_SOURCE_VIEW = Pattern.compile(
             "(?ms)^\\{\\{\\{code:([^\\r\\n]*)(?:\\r\\n?|\\n)(.*?)^\\}\\}\\}[ \\t]*$");
@@ -270,7 +272,7 @@ private String convertImage(final File file, final Context context, final String
     protected boolean isFileOutOfThisFormat(final File file, final String name, final String ext) {
         if (ext.equals(".txt")) {
             try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
-                return WikitextSyntaxHighlighter.ZIMHEADER_CONTENT_TYPE_ONLY.matcher(reader.readLine()).find();
+                return ZIMHEADER_CONTENT_TYPE_ONLY.matcher(reader.readLine()).find();
             } catch (Exception ignored) {
             }
         }
diff --git a/app/src/main/java/net/gsantner/markor/frontend/NewFileDialog.java b/app/src/main/java/net/gsantner/markor/frontend/NewFileDialog.java
index f27cc7f916..431636aa65 100644
--- a/app/src/main/java/net/gsantner/markor/frontend/NewFileDialog.java
+++ b/app/src/main/java/net/gsantner/markor/frontend/NewFileDialog.java
@@ -306,7 +306,7 @@ public void onTextChanged(CharSequence s, int start, int before, int count) {
             final File file = new File(basedir, fileName);
 
             // Most of the logic we want is in the document class so we just reuse it
-            final Document document = new Document(file);
+            final Document document = new Document(file, fmt.format);
 
             // These are done even if the file isn't created
             final String titleFormat = formatEdit.getText().toString().trim();
diff --git a/app/src/main/java/net/gsantner/markor/model/Document.java b/app/src/main/java/net/gsantner/markor/model/Document.java
index 6482b72102..2fac011455 100644
--- a/app/src/main/java/net/gsantner/markor/model/Document.java
+++ b/app/src/main/java/net/gsantner/markor/model/Document.java
@@ -71,10 +71,7 @@ public class Document implements Serializable {
     private int _lastLength = -1;
 
     public Document(@NonNull final File f) {
-        path = GsFileUtils.getPath(f);
-        file = new File(path);
-        title = GsFileUtils.getFilenameWithoutExtension(file);
-        extension = GsFileUtils.getFilenameExtension(file);
+        this(f, FormatRegistry.FORMAT_UNKNOWN);
 
         // Set initial format
         for (final FormatRegistry.Format format : FormatRegistry.FORMATS) {
@@ -85,6 +82,17 @@ public Document(@NonNull final File f) {
         }
     }
 
+    /**
+     * Create a document with a known format, skipping automatic format detection.
+     */
+    public Document(@NonNull final File f, @StringRes final int format) {
+        path = GsFileUtils.getPath(f);
+        file = new File(path);
+        title = GsFileUtils.getFilenameWithoutExtension(file);
+        extension = GsFileUtils.getFilenameExtension(file);
+        setFormat(format);
+    }
+
     private void initModTimePref() {
         // We do not do this in constructor as we want to init after deserialization too
         if (_modTimePref == null) {
diff --git a/app/src/test/java/net/gsantner/markor/format/wikitext/WikitextFileTests.java b/app/src/test/java/net/gsantner/markor/format/wikitext/WikitextFileTests.java
index 2f443f718b..5f71648284 100644
--- a/app/src/test/java/net/gsantner/markor/format/wikitext/WikitextFileTests.java
+++ b/app/src/test/java/net/gsantner/markor/format/wikitext/WikitextFileTests.java
@@ -57,6 +57,14 @@ public void zimHeaderAtBeginningOfTheFileShouldMatch() {
             assertThat(matcher.group()).isEqualTo("Content-Type: text/x-zim-wiki\nWiki-Format: zim 0.4\nCreation-Date: 2019-03-31T14:48:06+02:00");
         }
 
+        @Test
+        public void zimHeaderWithNegativeTimezoneOffsetShouldMatch() {
+            pattern = WikitextSyntaxHighlighter.ZIMHEADER;
+            Matcher matcher = pattern.matcher("Content-Type: text/x-zim-wiki\nWiki-Format: zim 0.4\nCreation-Date: 2019-03-31T14:48:06-07:00\nOther content...");
+            assertThat(matcher.find()).isTrue();
+            assertThat(matcher.group()).isEqualTo("Content-Type: text/x-zim-wiki\nWiki-Format: zim 0.4\nCreation-Date: 2019-03-31T14:48:06-07:00");
+        }
+
         @Test
         public void zimHeaderNotAtBeginningOfTheFileShouldNotMatch() {
             pattern = WikitextSyntaxHighlighter.ZIMHEADER;
diff --git a/app/src/test/java/net/gsantner/markor/model/DocumentConstructionTests.java b/app/src/test/java/net/gsantner/markor/model/DocumentConstructionTests.java
new file mode 100644
index 0000000000..1758b42497
--- /dev/null
+++ b/app/src/test/java/net/gsantner/markor/model/DocumentConstructionTests.java
@@ -0,0 +1,25 @@
+/*#######################################################
+ *
+ * SPDX-FileCopyrightText: 2026 Harshad Vedartham
+ * SPDX-License-Identifier: Apache-2.0
+ *
+#########################################################*/
+package net.gsantner.markor.model;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import net.gsantner.markor.format.FormatRegistry;
+
+import org.junit.Test;
+
+import java.io.File;
+
+public class DocumentConstructionTests {
+
+    @Test
+    public void knownFormatConstructorSetsFormatForAmbiguousFilename() {
+        final Document document = new Document(new File("archive.txt"), FormatRegistry.FORMAT_TODOTXT);
+
+        assertThat(document.getFormat()).isEqualTo(FormatRegistry.FORMAT_TODOTXT);
+    }
+}