MIME type validation when uploading file via REST API’s

Official description: Only allow image MIME type upload via REST APIs.

This vulnerability affects the following application versions:

• WooCommerce 2.5.0
• WooCommerce 2.5.0-RC1
• WooCommerce 2.5.0-RC2
• WooCommerce 2.5.0-RC3
• WooCommerce 2.5.1
• WooCommerce 2.5.2
• WooCommerce 2.5.3
• WooCommerce 2.5.4
• WooCommerce 2.5.5
• WooCommerce 2.6.0
• WooCommerce 2.6.0-RC2
• WooCommerce 2.6.1
• WooCommerce 2.6.2
• WooCommerce 2.6.3