Okay so this is weird… I sent a mobi file and it didn’t work.
Dear Customer,
A document sent at 10:24 AM on Wed, Dec 13, 2023 GMT could not be delivered.
Send to Kindle can convert and deliver the following types of documents: • EPUB (.epub) • Adobe PDF (.pdf) • Microsoft Word (.doc, .docx) • Rich Text Format (.rtf) • HTML (.htm, .html) • Text (.txt) documents • Compressed documents (.zip, .xzip) • JPEG (.jpg), GIF (.gif), Bitmap (.bmp) and PNG (.png) images.