Thanks for posting.
This appears to be something that the Chrome browser is doing. I suspect that it's merely changing the extension rather than converting the file.
If you download the file in another browser (e.g., Firefox), it comes through with its original name.
I can assure you that SendNow is not doing any file conversion.
Thanks for the additional information. I have filed a bug with the team to see if there is anything that we can do on our end to try to stop Chrome and IE9 from changing the extension. I'll keep you updated on what I learn.
It looks like .mp4 files are being served with Content-Type: video/quicktime, instead of Content-Type: video/mp4, which may explain why Chrome (and IE) are doing the "right" thing and changing the extension to reflect the erroneous Content-Type.