diff --git a/assets/js/single-attachment.js b/assets/js/single-attachment.js
index f2fc68255..d30a9adc6 100644
--- a/assets/js/single-attachment.js
+++ b/assets/js/single-attachment.js
@@ -125,6 +125,7 @@ jQuery(document).ready(function($) {
   function uploadFile() {
     var formData = new FormData();
     formData.append("action", "optml_replace_file");
+    formData.append("optml_replace_nonce", OMAttachmentEdit.nonce);
     formData.append("attachment_id", OMAttachmentEdit.attachmentId);
     formData.append("file", $("#optml-replace-file-field")[0].files[0]);
 
@@ -138,11 +139,12 @@ jQuery(document).ready(function($) {
       contentType: false,
       success: function(response) {
         $(".optml-svg-loader").hide();
-        if(response.success) {
+        if (response && response.success) {
           window.location.reload();
         } else {
+          var msg = (response && (response.data || response.message)) || OMAttachmentEdit.i18n.replaceFileError;
           $(".optml-replace-file-error").removeClass("hidden");
-          $(".optml-replace-file-error").text(response.message);
+          $(".optml-replace-file-error").text(msg);
         }
       },
       error: function(response) {
diff --git a/inc/media_rename/attachment_edit.php b/inc/media_rename/attachment_edit.php
index 0663df3bf..86bc4bff1 100644
--- a/inc/media_rename/attachment_edit.php
+++ b/inc/media_rename/attachment_edit.php
@@ -92,6 +92,7 @@ public function enqueue_scripts( $hook ) {
 					'maxFileSize'  => $max_file_size,
 					'attachmentId' => $id,
 					'mimeType'     => $mime_type,
+					'nonce'        => wp_create_nonce( 'optml_replace_media_nonce' ),
 					'i18n'         => [
 						'maxFileSizeError' => $max_file_size_error,
 						'replaceFileError' => __( 'Error replacing file', 'optimole-wp' ),
@@ -329,7 +330,19 @@ public function save_attachment_filename( $post_id ) {
 	 * Replace the file
 	 */
 	public function replace_file() {
-		$id = (int) sanitize_text_field( $_POST['attachment_id'] );
+		if ( ! check_ajax_referer( 'optml_replace_media_nonce', 'optml_replace_nonce', false ) ) {
+			wp_send_json_error( __( 'Security check failed', 'optimole-wp' ) );
+		}
+
+		$id = absint( $_POST['attachment_id'] ?? 0 );
+
+		if ( ! $id ) {
+			wp_send_json_error( __( 'Invalid attachment ID', 'optimole-wp' ) );
+		}
+
+		if ( get_post_type( $id ) !== 'attachment' ) {
+			wp_send_json_error( __( 'Invalid attachment ID', 'optimole-wp' ) );
+		}
 
 		if ( ! current_user_can( 'edit_post', $id ) ) {
 			wp_send_json_error( __( 'You are not allowed to replace this file', 'optimole-wp' ) );
@@ -339,6 +352,17 @@ public function replace_file() {
 			wp_send_json_error( __( 'No file uploaded', 'optimole-wp' ) );
 		}
 
+		$file_info = wp_check_filetype_and_ext( $_FILES['file']['tmp_name'], $_FILES['file']['name'] );
+
+		if ( empty( $file_info['type'] ) ) {
+			wp_send_json_error( __( 'Could not determine uploaded file type', 'optimole-wp' ) );
+		}
+
+		$original_mime = get_post_mime_type( $id );
+		if ( $file_info['type'] !== $original_mime ) {
+			wp_send_json_error( __( 'The uploaded file type does not match the original file type.', 'optimole-wp' ) );
+		}
+
 		$replacer = new Optml_Attachment_Replace( $id, $_FILES['file'] );
 
 		$replaced = $replacer->replace();
diff --git a/tests/media_rename/test-attachment-edit.php b/tests/media_rename/test-attachment-edit.php
index 87e43bcc7..46a6e0e5d 100644
--- a/tests/media_rename/test-attachment-edit.php
+++ b/tests/media_rename/test-attachment-edit.php
@@ -5,8 +5,14 @@
 
 /**
  * Class Test_Attachment_Edit.
+ *
+ * Extends WP_Ajax_UnitTestCase so that:
+ * - DOING_AJAX is true, ensuring wp_send_json_* calls wp_die() rather than die()
+ * - wp_die() is overridden to throw WPAjaxDieContinueException / WPAjaxDieStopException
+ *   after storing the response in $this->_last_response
+ * - _handleAjax() dispatches wp_ajax_* actions and captures JSON output cleanly
  */
-class Test_Attachment_Edit extends WP_UnitTestCase {
+class Test_Attachment_Edit extends WP_Ajax_UnitTestCase {
 	/**
 	 * Test instance
 	 *
@@ -14,12 +20,50 @@ class Test_Attachment_Edit extends WP_UnitTestCase {
 	 */
 	private $instance;
 
+	/**
+	 * A real JPEG attachment (file on disk) shared across replace_file tests.
+	 *
+	 * @var int
+	 */
+	private static $jpeg_attachment_id;
+
+	/**
+	 * Create a real JPEG attachment once for the whole class.
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$jpeg_attachment_id = $factory->attachment->create_upload_object(
+			OPTML_PATH . 'tests/assets/sample-test.jpg'
+		);
+	}
+
+	/**
+	 * Clean up the shared attachment after all tests in this class run.
+	 */
+	public static function tear_down_after_class() {
+		wp_delete_post( self::$jpeg_attachment_id, true );
+		parent::tear_down_after_class();
+	}
+
 	/**
 	 * Setup test
 	 */
 	public function setUp(): void {
 		parent::setUp();
+		wp_set_current_user( 1 );
+
 		$this->instance = new Optml_Attachment_Edit();
+
+		// Register the wp_ajax_optml_replace_file action so _handleAjax() can
+		// invoke replace_file() through the normal WordPress AJAX dispatch path.
+		$this->instance->init();
+	}
+
+	/**
+	 * Reset $_FILES after each test (WP_Ajax_UnitTestCase resets $_POST/$_GET).
+	 */
+	public function tearDown(): void {
+		$_FILES = [];
+		parent::tearDown();
 	}
 
 	/**
@@ -49,4 +93,196 @@ public function test_prepare_attachment_filename() {
 
 		$this->assertEquals( 'test-file', get_post_meta( $attachment->ID, '_optml_pending_rename', true ) );
 	}
+
+	/**
+	 * Dispatch the optml_replace_file AJAX action with the given POST data and
+	 * $_FILES, catch the wp_die() exception, and return the decoded JSON.
+	 *
+	 * _handleAjax() starts its own output buffer and the die handler stores
+	 * the captured JSON in $this->_last_response before throwing, so no manual
+	 * ob_start() is needed here.
+	 *
+	 * @param array $post  Contents for $_POST (nonce and attachment_id).
+	 * @param array $files Contents for $_FILES['file'], or empty to omit.
+	 * @return array Decoded JSON response array, or empty array.
+	 */
+	private function call_replace_file( array $post, array $files = [] ): array {
+		$_POST  = $post;
+		$_FILES = $files;
+
+		try {
+			$this->_handleAjax( 'optml_replace_file' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			// Normal path: wp_send_json_* echoed JSON then called wp_die().
+		} catch ( WPAjaxDieStopException $e ) {
+			// Fallback: wp_die() called with a plain string (e.g. '-1' from a
+			// nonce failure when $die = true). Treat as a failed response.
+		}
+
+		return ! empty( $this->_last_response )
+			? ( json_decode( $this->_last_response, true ) ?? [] )
+			: [];
+	}
+
+	/**
+	 * A request with no nonce must be rejected with a security error.
+	 */
+	public function test_replace_file_no_nonce() {
+		$response = $this->call_replace_file( [ 'attachment_id' => (string) self::$jpeg_attachment_id ] );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'Security check', $response['data'] );
+	}
+
+	/**
+	 * A request with a syntactically valid but wrong nonce must be rejected.
+	 */
+	public function test_replace_file_bad_nonce() {
+		$response = $this->call_replace_file( [
+			'attachment_id'       => (string) self::$jpeg_attachment_id,
+			'optml_replace_nonce' => 'not_a_real_nonce',
+		] );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'Security check', $response['data'] );
+	}
+
+	/**
+	 * attachment_id = 0 must be rejected before any file or capability check.
+	 */
+	public function test_replace_file_zero_id() {
+		$response = $this->call_replace_file( [
+			'attachment_id'       => '0',
+			'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+		] );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'Invalid attachment ID', $response['data'] );
+	}
+
+	/**
+	 * An ID that belongs to a non-attachment post type must be rejected.
+	 */
+	public function test_replace_file_non_attachment_post() {
+		$post_id = self::factory()->post->create( [ 'post_type' => 'post' ] );
+
+		$response = $this->call_replace_file( [
+			'attachment_id'       => (string) $post_id,
+			'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+		] );
+
+		wp_delete_post( $post_id, true );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'Invalid attachment ID', $response['data'] );
+	}
+
+	/**
+	 * A request with no $_FILES['file'] entry must be rejected.
+	 */
+	public function test_replace_file_no_file_uploaded() {
+		$response = $this->call_replace_file( [
+			'attachment_id'       => (string) self::$jpeg_attachment_id,
+			'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+		] );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'No file uploaded', $response['data'] );
+	}
+
+	/**
+	 * A binary blob named .jpg whose content is not a real image must be
+	 * rejected: wp_check_filetype_and_ext() uses getimagesize() for image/*
+	 * types and returns an empty type when the magic bytes do not match.
+	 */
+	public function test_replace_file_unrecognizable_mime() {
+		$tmp = tempnam( sys_get_temp_dir(), 'optml_test_' );
+		file_put_contents( $tmp, 'this is not image data @@##$$%%' );
+
+		$response = $this->call_replace_file(
+			[
+				'attachment_id'       => (string) self::$jpeg_attachment_id,
+				'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+			],
+			[
+				'file' => [
+					'name'     => 'fake.jpg',
+					'type'     => 'image/jpeg',
+					'tmp_name' => $tmp,
+					'error'    => 0,
+					'size'     => filesize( $tmp ),
+				],
+			]
+		);
+
+		@unlink( $tmp );
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'determine', $response['data'] );
+	}
+
+	/**
+	 * Uploading a file whose real MIME type differs from the original attachment
+	 * (SVG against a JPEG attachment) must be rejected by the MIME-match check.
+	 */
+	public function test_replace_file_mime_mismatch() {
+		$response = $this->call_replace_file(
+			[
+				'attachment_id'       => (string) self::$jpeg_attachment_id,
+				'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+			],
+			[
+				'file' => [
+					'name'     => 'sample.svg',
+					'type'     => 'image/svg+xml',
+					'tmp_name' => OPTML_PATH . 'tests/assets/sample.svg',
+					'error'    => 0,
+					'size'     => filesize( OPTML_PATH . 'tests/assets/sample.svg' ),
+				],
+			]
+		);
+
+		$this->assertFalse( $response['success'] );
+		$this->assertStringContainsString( 'does not match', $response['data'] );
+	}
+
+	/**
+	 * A valid nonce, valid attachment ID, and a replacement whose MIME type
+	 * matches the original must pass all validation and succeed.
+	 *
+	 * A fresh attachment is created so the shared fixture is not consumed by
+	 * the actual file-move that Optml_Attachment_Replace performs.
+	 */
+	public function test_replace_file_valid_jpeg_replacement() {
+		$attachment_id = self::factory()->attachment->create_upload_object(
+			OPTML_PATH . 'tests/assets/sample-test.jpg'
+		);
+
+		// Optml_Attachment_Replace moves (not copies) the tmp file.
+		$tmp = tempnam( sys_get_temp_dir(), 'optml_repl_' ) . '.jpg';
+		copy( OPTML_PATH . 'tests/assets/small-1.jpg', $tmp );
+
+		$response = $this->call_replace_file(
+			[
+				'attachment_id'       => (string) $attachment_id,
+				'optml_replace_nonce' => wp_create_nonce( 'optml_replace_media_nonce' ),
+			],
+			[
+				'file' => [
+					'name'     => 'small-1.jpg',
+					'type'     => 'image/jpeg',
+					'tmp_name' => $tmp,
+					'error'    => 0,
+					'size'     => filesize( $tmp ),
+				],
+			]
+		);
+
+		if ( file_exists( $tmp ) ) {
+			@unlink( $tmp );
+		}
+		wp_delete_post( $attachment_id, true );
+
+		$this->assertTrue( $response['success'] );
+	}
 }