diff --git a/boards/templates/boards/all_threads.html b/boards/templates/boards/all_threads.html
--- a/boards/templates/boards/all_threads.html
+++ b/boards/templates/boards/all_threads.html
@@ -163,8 +163,8 @@
                 {{ form.as_div }}
                 <div class="form-submit">
                     <input type="submit" value="{% trans "Post" %}"/>
-                    <button id="preview-button" onclick="return false;">{% trans 'Preview' %}</button>
-                    <button id="file-source-button" onclick="return false;">{% trans 'Change file source' %}</button>
+                    <button id="preview-button" type="button" onclick="return false;">{% trans 'Preview' %}</button>
+                    <button id="file-source-button" type="button" onclick="return false;">{% trans 'Change file source' %}</button>
                 </div>
             </form>
             </div>
diff --git a/boards/templates/boards/thread_normal.html b/boards/templates/boards/thread_normal.html
--- a/boards/templates/boards/thread_normal.html
+++ b/boards/templates/boards/thread_normal.html
@@ -53,8 +53,8 @@
                         {{ form.as_div }}
                         <div class="form-submit">
                             <input type="submit" value="{% trans "Post" %}"/>
-                            <button id="preview-button" onclick="return false;">{% trans 'Preview' %}</button>
-                            <button id="file-source-button" onclick="return false;">{% trans 'Change file source' %}</button>
+                            <button id="preview-button" type="button" onclick="return false;">{% trans 'Preview' %}</button>
+                            <button id="file-source-button" type="button" onclick="return false;">{% trans 'Change file source' %}</button>
                         </div>
                     </form>
                 </div>