Update manipulating-content-the-slingpostservlet-servlets-post.md

[gauravs23]

Update node name and node type generation algorithm description in case of File Uploads using SlingPostServlet

[bdelacretaz]

The proposed changes look good to me in principle, thank you very much! But do we have tests of those features? I don't remember the details off the top of my head and I'd like to make sure those details are correct. Or maybe have less details in the docs and point to those tests for the details.