@aaarrrgh - you can use the Content-Disposition: attachment to kinda force the user to download the file instead of displaying in the browser. Useful for PDF files.
@hantaah - The safari mobile browser won't download - it'll just attempt to display in the browser with a bar on top letting you choose if you wanna open in dropbox (or other applications that have been registered with the iOS to open files, i guess). if its not something readable, it'll just display an icon.
On mobile chrome, there is no bar, if its not something readable by the browser, it just shows that chrome is unable to download the file.