The file type .mobi is an eBook format created by a French eBook company called Mobipocket which created a free software application for multiple devices including PDAs, smartphones and tablet devices. The company was bought by Amazon in 2005 and the support of the .mobi file extension was officially discontinued in 2011 with the AZW format superseding it.
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AZW3 - Kindle Format 8 (KF8) is Amazon's newer version of AZW. It supports HTML5 and CSS3 which were not supported through the standard AZW format. It has a number of other formatting options and is used by Amazon for all new Amazon eBooks. It is supported by fourth-generation Kindle devices running firmware version 4.1.0 or later and the Kindle Fire device.
KF8 (also called AZW3) is essentially a compiled ePub that has utlises a Palm database and Amazon's DRM scheme. It targets ePub 3 version support but existing ePub can also be used. The internal format has been decoded using mobi_unpack. The basic format remains as a PDB file similar in structure to a MOBI. More oftent than not a KF8 file will contain both a MOBI file at the beginning and the newer KF8 version of ePub later.
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