Default directory for browser attachments

I am using Facebook on Chrome and often have to attach files using FB's chat interface. When attaching photos, Caja opens up to my home folder. I want Caja to open my Desktop by default whenever I am attaching files. Is there anyway to do that?

As much as it may seem to the contrary, what provides the Open Files... dialog box is not Caja; it is a part of the graphical user interface library known as GTK+. The file chooser dialog is actually fashioned to look like GNOME's Nautilus file manager, but it is not Caja.

I don't use Chrome or a derivative browser, so I can't help very much in that regard; however, try closing down Chrome completely, then opening the terminal application (search in the menu for Terminal) and type:

cd Desktop && chrome

If subsequently opening the file chooser opens up to your ~/Desktop directory by default after running that command, then maybe you can edit your launcher for Chrome: Right-click on the launcher button that you use (if you click a button to launch Chrome), or open the menu editor (search the menu for Menu Editor) and select the entry for Chrome. Either way, there should be a choice to "edit" the launcher; click that choice. Change the Application string in the dialog that pops up to:

sh -c 'cd ~/Desktop; chrome %u'

Click OK to dismiss the dialog and save your changes. That should fix the launcher.

Sorry for my terseness -- I'm in a hurry right now.