How to use post formats with MarsEdit — WordPress
MarsEdit has one deficiency: there is no native support for setting Post Formats. […]
An hour of code later, I had a very simple plugin to solve this problem in MarsEdit. You can set a custom field to check, and a post format you’d like to change to.
Every time you create or update a post, it’ll check if you have a value set for that custom field. If you do, it’ll set the post format to your desired format.
Even if the plugin has not been updated in almost two years, it still works! I used it to publish this story. A great work by Marcelo Somers.