I understand that there are sites that allow free downloads of Kindle ebooks. How does someone in possession of a mobi Kindle file even get it off the Kindle and then share it? I'm sure Amazon would cry foul.
I dont know kindle, but every portable devices that I have fooled with have had all of their their files accessable directly while attached to a computer... from myComputer explore.
Can simply copy/cut/paste/move... etc.
A basic Google search using the keywords "Kindle forum" (sans quotes) revealed many instances of Kindle forums, both Amazon based and independent, which may be a better place to research or ask your Kindle question. At those forums members' participation is dedicated to the unit, and they should be more prepared to answer any question pertaining to it.
Kindle and Amazon are fine with the use of Mobi files it is made to be able to use Mobi.
My family that have Kindles and use the kindle on their phones all use Mobi version back and forth.
Since I do not have a kindle I can not tell you exactly the steps but I am sure any of the Kindle forums would have that info.
Take a look at Kindleboards. The link below is for a search I did for you using the term "mobi". If that doesn't answer your question, then just post it there....
Here is a link that might be useful: Kindleboards: Mobi files