diff -r 48c4eec2b7e6 -r 8c2e4d02f4ef wp/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
--- a/wp/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	Fri Sep 05 18:40:08 2025 +0200
+++ b/wp/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	Fri Sep 05 18:52:52 2025 +0200
@@ -134,6 +134,44 @@
 				array( 'status' => rest_authorization_required_code() )
 			);
 		}
+		$files = $request->get_file_params();
+
+		/**
+		 * Filter whether the server should prevent uploads for image types it doesn't support. Default true.
+		 *
+		 * Developers can use this filter to enable uploads of certain image types. By default image types that are not
+		 * supported by the server are prevented from being uploaded.
+		 *
+		 * @since 6.8.0
+		 *
+		 * @param bool        $check_mime Whether to prevent uploads of unsupported image types.
+		 * @param string|null $mime_type  The mime type of the file being uploaded (if available).
+		 */
+		$prevent_unsupported_uploads = apply_filters( 'wp_prevent_unsupported_mime_type_uploads', true, isset( $files['file']['type'] ) ? $files['file']['type'] : null );
+
+		// If the upload is an image, check if the server can handle the mime type.
+		if (
+			$prevent_unsupported_uploads &&
+			isset( $files['file']['type'] ) &&
+			str_starts_with( $files['file']['type'], 'image/' )
+		) {
+			// List of non-resizable image formats.
+			$editor_non_resizable_formats = array(
+				'image/svg+xml',
+			);
+
+			// Check if the image editor supports the type or ignore if it isn't a format resizable by an editor.
+			if (
+				! in_array( $files['file']['type'], $editor_non_resizable_formats, true ) &&
+				! wp_image_editor_supports( array( 'mime_type' => $files['file']['type'] ) )
+			) {
+				return new WP_Error(
+					'rest_upload_image_type_not_supported',
+					__( 'The web server cannot generate responsive image sizes for this image. Convert it to JPEG or PNG before uploading.' ),
+					array( 'status' => 400 )
+				);
+			}
+		}
 
 		return true;
 	}
@@ -339,8 +377,7 @@
 		 *
 		 * @since 4.7.0
 		 *
-		 * @param WP_Post         $attachment Inserted or updated attachment
-		 *                                    object.
+		 * @param WP_Post         $attachment Inserted or updated attachment object.
 		 * @param WP_REST_Request $request    The request sent to the API.
 		 * @param bool            $creating   True when creating an attachment, false when updating.
 		 */
@@ -450,7 +487,7 @@
 	}
 
 	/**
-	 * Performs post processing on an attachment.
+	 * Performs post-processing on an attachment.
 	 *
 	 * @since 5.3.0
 	 *
@@ -471,7 +508,7 @@
 	}
 
 	/**
-	 * Checks if a given request can perform post processing on an attachment.
+	 * Checks if a given request can perform post-processing on an attachment.
 	 *
 	 * @since 5.3.0
 	 *
@@ -531,7 +568,7 @@
 			);
 		}
 
-		$supported_types = array( 'image/jpeg', 'image/png', 'image/gif', 'image/webp', 'image/avif' );
+		$supported_types = array( 'image/jpeg', 'image/png', 'image/gif', 'image/webp', 'image/avif', 'image/heic' );
 		$mime_type       = get_post_mime_type( $attachment_id );
 		if ( ! in_array( $mime_type, $supported_types, true ) ) {
 			return new WP_Error(
@@ -601,7 +638,7 @@
 			$args = $modifier['args'];
 			switch ( $modifier['type'] ) {
 				case 'rotate':
-					// Rotation direction: clockwise vs. counter clockwise.
+					// Rotation direction: clockwise vs. counterclockwise.
 					$rotate = 0 - $args['angle'];
 
 					if ( 0 !== $rotate ) {
@@ -661,7 +698,7 @@
 
 		$filename = "{$image_name}.{$image_ext}";
 
-		// Create the uploads sub-directory if needed.
+		// Create the uploads subdirectory if needed.
 		$uploads = wp_upload_dir();
 
 		// Make the file name unique in the (new) upload directory.
@@ -1207,7 +1244,7 @@
 				continue;
 			}
 
-			list( $type, $attr_parts ) = explode( ';', $value, 2 );
+			list( , $attr_parts ) = explode( ';', $value, 2 );
 
 			$attr_parts = explode( ';', $attr_parts );
 			$attributes = array();