https://docs.magnolia-cms.com/product-docs/6.2/Developing/Templating/Dialog-definition/Field-definition/List-of-fields/Upload-field.html

One can now specify also list of extensions for given mimeType in the list. This is needed (exclusively?) for 'application/x-yaml' mime type which is not working properly in the field, hence 'application/x-yaml,.yaml' is now possible configuration to workaround this.