I'm having the same issue. I can upload a pdf, but the mobi file fails. I have tested it repeatedly, from two different locations - dropbox and my local hard drive. It appears to work, but once it's done the software still reports "Ebook file is still not uploaded." I thought about just uploading it to the uploads directory, but I don't know if the software has to register it and won't recognize a straight upload.
UPDATE: I just uploaded the file manually, and the software still doesn't recognize that it exists on the hard drive. Then I reran the upload process to see if it would recognize that the file is there. It did not. During the upload process the progress of the upload, in percentages, is indicated in the lower left hand corner of the screen, but the software never uploads it and doesn't recognize that it's already there.
