Nearly every browser will show image oder video files you want to provide for download in the browser itself instead of opening a download dialog.

Most of the websites suggest to use this code in your .htaccess:

<FilesMatch "\.(jpg|zip|avi)$">
  ForceType application/octet-stream
</FilesMatch>

But this is not enough! Even when the MIME-Type is set to "octet-stream", some browsers will still open the files their selfs because they have detected the .jpg or .avi file extension.

The solution to this is to add the following header:

<FilesMatch "\.(jpg|zip|avi)$">
  ForceType application/octet-stream
  Header add Content-Disposition "attachment"
</FilesMatch>

With this header every browser i tested opened a download dialog, regardless of which file extension is present.

To use this, your webserver has to have mod_headers activated (It is most likely already included in Apache, but not activated): Apache Module mod_headers