What determines, and can it be changed, when for example, in Firefox, you have some image, and you click on View image, and it options for download while sometimes he just displays the image?
I'd like it to display the image every time.
Content-Type
and/or Content-Disposition
headers (sent from the webserver) determine whether Firefox "dares" to show a file in-line. You can use firebug or tamperdata extensions to view these headers (or sniff them with wireshark if your source is not https).
You can show files in <img>
tags inline even if they use a nonsupported content-type...
<img src="http://20.imagebam.com/dl.php?ID=52223127&sec=f55ae9365166a422086dd85c1b35b0cf">
works for me. You cannot link to the image inline because it sets Content-Disposition=attachment. Store the image elsewhere if you need to link to the image inline.