Actually, I didn't verify if .mobi is allowed or not. I used .docx and didn't check beyond that. Your comment made me go back to look. It appears that .mobi isn't allowed.
See here:
https://kdp.amazon.com/help?topicId=A39QXS5Z54LMXF
If you can't access the link, just so you know, the only files mentioned are PDF, DOC (.doc), DOCX (.docx), HTML (.html), or RTF (.rtf). I'm not exactly an expert though, so don't take my word for it.