Can you upload other file types such as docx, pdf, jpg, etc? If yes, then your epub file may be DRM protected and that's why Yahoo doesn't allow the file to be sent or saved as a draft.
If it isn't DRM protected but that Yahoo won't attach your file, it may be possible that epub files aren't allowed. You can try with a different DRM-free epub file for confirmation.
Alternatively, I suggest you try Send to Kindle to send your file.