diff --git a/webapp/migrations/Version20231120225210.php b/webapp/migrations/Version20231120225210.php
new file mode 100644
index 0000000000..4f2bedd3cc
--- /dev/null
+++ b/webapp/migrations/Version20231120225210.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20231120225210 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Remove leading dots from extensions in the `language` table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $languages = $this->connection->fetchAllAssociative('SELECT langid, extensions FROM language');
+
+        foreach ($languages as $language) {
+            $extensions = json_decode($language['extensions'], true);
+
+            $updated = false;
+            foreach ($extensions as &$extension) {
+                if (strpos($extension, '.') === 0) {
+                    $extension = ltrim($extension, '.');
+                    $updated = true;
+                }
+            }
+
+            if ($updated) {
+                $newExtensionsJson = json_encode($extensions);
+                $this->connection->executeQuery('UPDATE language SET extensions = :extensions WHERE langid = :langid', [
+                    'extensions' => $newExtensionsJson,
+                    'langid' => $language['langid'],
+                ]);
+            }
+        }
+    }
+
+    public function down(Schema $schema): void
+    {
+        // This migration is not reversible
+    }
+
+    public function isTransactional(): bool
+    {
+        return false;
+    }
+}
diff --git a/webapp/src/Entity/Language.php b/webapp/src/Entity/Language.php
index 2ff25e0e0d..4d2e76627e 100644
--- a/webapp/src/Entity/Language.php
+++ b/webapp/src/Entity/Language.php
@@ -53,6 +53,12 @@ class Language extends BaseApiEntity
         options: ['comment' => 'List of recognized extensions (JSON encoded)']
     )]
     #[Assert\NotBlank]
+    #[Assert\All([
+        new Assert\Regex([
+            'pattern' => '/^[^.]/',
+            'message' => 'The extension should not start with a dot.'
+        ])
+    ])]
     #[Serializer\Type('array<string>')]
     private array $extensions = [];
 
diff --git a/webapp/src/Service/SubmissionService.php b/webapp/src/Service/SubmissionService.php
index 73f656e9e5..c53cd17e45 100644
--- a/webapp/src/Service/SubmissionService.php
+++ b/webapp/src/Service/SubmissionService.php
@@ -530,8 +530,7 @@ public function submitSolution(
             if ($source !== 'shadowing' && $language->getFilterCompilerFiles()) {
                 $matchesExtension = false;
                 foreach ($language->getExtensions() as $extension) {
-                    $extensionLength = strlen($extension);
-                    if (substr($file->getClientOriginalName(), -$extensionLength) === $extension) {
+                    if (str_ends_with($file->getClientOriginalName(), '.' . $extension)) {
                         $matchesExtension = true;
                         break;
                     }
diff --git a/webapp/templates/jury/partials/language_form.html.twig b/webapp/templates/jury/partials/language_form.html.twig
index 82e9d7b1ad..7409b3958f 100644
--- a/webapp/templates/jury/partials/language_form.html.twig
+++ b/webapp/templates/jury/partials/language_form.html.twig
@@ -18,7 +18,7 @@
             addExtension($extensionsHolder, $addExtensionButton);
         });
 
-        $extensionsHolder.find('div').each(function() {
+        $extensionsHolder.find('div:not(.invalid-feedback)').each(function() {
             addDeleteLink($(this));
         });
 
@@ -37,8 +37,10 @@
             var $removeFormButton = $('<button type="button" class="btn btn-danger"><i class="fas fa-trash"></i></button>');
             var $inputGroup = $('<div class="input-group"></div>');
             var $formControl = $extensionDiv.find('.form-control');
+            var $feedbackBlock = $extensionDiv.find('.invalid-feedback');
             $inputGroup.append($formControl);
             $inputGroup.append($removeFormButton);
+            $inputGroup.append($feedbackBlock);
             $extensionDiv.html($inputGroup);
 
             $removeFormButton.on('click', function(e) {