The suffix tells us exactly what we are looking at: a Mobipocket eBook file. This format was originally designed for the Mobipocket Reader but became famous as the primary format for Amazon Kindle devices before they transitioned toward .AZW3 and .KFX.
In some cases, these strings contain encoded information about the publisher or the specific edition of the text. How to Open a .Mobi File
You can send .mobi files directly to your Kindle via USB or the "Send to Kindle" service.