Hi donkeymusic
There's no ".dtp" file. What you're thinking of, I think, is Amazon's "Digital Text Platform" (DTP) - that's their
web-based interface for self-publishing ebooks for the Kindle.
Ebooks for the Kindle use the MOBI file format. However as Amoroso already indicated, Amazon's DTP interface will accept various types of input files, including EPUB files, and will convert them to the required MOBI format. When you set up the book details on the DTP website and upload the file, you will have an option to preview the converted file - the preview is a good indication of what the book will look like on a Kindle. How well the conversion works will depend on a range of issues, including how the book is formatted - there are things you can do in EPUB format that simply won't work in MOBI format.
The Community forums on the DTP website are a great source of advice and information on publishing with Amazon's DTP interface, if you run into any problems, donkeymusic.