We checked on your sitemap at https://www.railwayphotography.co.uk/sitemap_index.xml and currently it is loading for us as expected. We also checked the sitemap in an XML sitemap validator here and no issues were reported.
However, if you are still receiving the error of ‘Unsupported format. Your file appears to be an HTML page’ in Google Search Console, this can be caused by a number of issues, including the submitted URL, or that the sitemap page is being redirected. Can you please confirm whether the sitemap you submitted to Google is for ‘sitemap.xml’ or for ‘sitemap_index.xml’ ? ?If you have submitted the sitemap as ‘sitemap.xml’ you would need to delete that sitemap from your Google Search Console and then resubmit the sitemap as ‘sitemap_index.xml.’
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?We also have additional information on the error that your sitemap is not in the correct format and is an HTML file at the following link: https://kb.yoast.com/kb/why-is-google-saying-my-xml-video-sitemap-is-an-html-file/
