Since Amazon offers conversion on their site from an
.epub file to
.mobi file do they support fixed layout ePubs or is it only for KF8 files?
If you have a fixed layout epub 3.0 file, it's not much of a jump to prepare it for conversion to KF8 fixed layout. KF8 is, of course, the newest version of Amazon's proprietary ebook format. KindleGen or Kindle Previewer are the two options for creating a KF8 file from an epub file. Kindle Previewer is essentially a graphical interface for KindleGen that comes packaged with a previewer for the various Kindle devices. Personally, I prefer it over KindleGen; the previewing functionality is really handy. That said, don't trust the previewer too far--it will not always accurately represent what the devices will show. It's always best to test on an actual device, if you can.
The guidelines for preparing an epub file for KindleGen/Previewer are found here: https://kindlegen.s3.amazonaws.com/AmazonKindlePublishingGuidelines.pdf (PDF format) In general, having a validating epub 3.0 fixed layout file will get you 95% of the way there, though KF8 introduces some extra functionality like being able to define verso and recto (left and right) pages in a two-page spread, and also region magnification to allow zooming on particular areas of text. There is a sample KF8 fixed layout file available at http://www.amazon.com/gp/feature.html?docId=1000729511.
Again, be certain that the Fixed Layout epub that you have is the epub 3.0 variety, rather than Apple's iBooks-only fixed layout format. The Apple format came out before the IDPF developed Fixed Layout for epub 3.0, and while Apple's format seems to have served as the basis for the IDPF format, it doesn't necessarily make a valid epub 3.0 file. You can use the online validator here (http://validator.idpf.org/) to test your file to be sure that it's valid.
So, long story short: Amazon does not sell or support epub files of any type, but they do have tools and information available for converting valid epub files to mobi/KF8, including for Fixed Layout books.
Hope that helps!
It really depends what you mean by "support". If you package your EPUB according to the KDP guidelines (see section 4, "Creating Fixed Layout Children’s Books"), then KindleGen will convert it to MOBI. You can test it yourself, by installing KindleGen locally on your machine (the Kindle web site does nothing more than running it for you remotely, if you provide an EPUB file).
Probably, if you start from an EPUB FXL designed for e.g. iBooks, you will need to patch it for meeting the additional requirements of Amazon (in order to have KindleGen to convert it to KF8 successfully).