I created an ePub file that works fine. I used the Creative Cloud version of inDesign on my MacBook Pro with OS 10.7.
I emailed to someone to view. The recipient was going to download the ePub to her Mac and sync with her iPad.
She was using Safari and gmail. She received the email with the attachment. When she clicked download in the email next to the attachment, it downloaded but added the .flv extension to the file.
With this extension, the file could not be added to the iTunes library. Removing the .flv so that the filename was restored to the original with the .epub extension, all worked just fine.
I set all .epub files to be opened by Adobe Digital Editions on my computer.
I replicated the same situation on my Mac with Safari and gmail. Then, I tried with my yahoo email account. Same situation, file downloads but gets .flv extension added. Guess I can't blame gmail.
Tried with Chrome and file downloaded with correct name, no extra extension.
Tried with Entourage and file save with .epub, no extra extension.
Have not yet tried this on a windows machine but will do so in a bit and report back with results.
How can I prevent the extra extension?
Just checked this on a Windows 8 laptop.
Went to gmail. Opened the message and downloaded the ePub file. It downloaded just exactly as expected. No change to the file name.
I zipped up a folder with the ePub file inside and tried that zip file. I emailed the zip file to my gmail account and then downloaded the zip file and unzipped it. The ePub file came through the process with no alteration to its name.
As long as I send the ePub inside a zip, I don't seem to have a problem.
I would still like to know how to prevent the change in name when I email just the ePub file, though.