Description of problem:

If a user checks the "Do this automatically from now on" box when opening
a downloaded file with a helper application, there is no apparent way to
ever reverse that decision. The indicated action does not appear when
you click "View/Edit Actions" in the Downloads preferences -- indeed, that
list is always empty.

Version-Release number of selected component (if applicable):

1.5.0.9-2.fc6

How reproducible:

Always.

Steps to Reproduce:
1. Start with default firefox configuration on a new Fedora Core installation
(make sure user does not have a preexisting mimeTypes.rdf file).
2. Click on a link to a pdf file.
3. Select a helper application.
4. Check the "Do this automatically from now on" box.
5. Complete the download.
6. Go to Edit->Preferences, select Downloads tab, and click on View/Edit Actions.

Actual results:

You see an empty list, with no way to undo or reverse the selection of
automatically using that helper application for pdf files.

Expected results:

You should see a list containing your selected helper application for pdf files,
with the option to edit or remove that automatic action.

Additional info:

The reason is because GNOME has now deprecated all functions that return
extensions for MIME types, and firefox is configured by default to hide all
actions for MIME types without extensions.

Firefox calls gnome_vfs_mime_get_extensions_list (in 
mozilla/uriloader/exthandler/unix/nsGNOMERegistry.cpp) to get the list of
extensions for a MIME type. However, in current versions of GNOME that function
is hardcoded to always return NULL, so all MIME types now have an empty
extension list in firefox (except for ones left over in a user's mimeTypes.rdf
file from running previous versions of firefox with earlier versions of GNOME).

The firefox configuration by default sets the
"browser.download.hide_plugins_without_extensions" preference (which applies to
helper apps as well as to plugins). Therefore, all actions are hidden and the
user cannot reverse or undo the action.

I have attached a patch which alleviates the problem by removing the default
setting of browser.download.hide_plugins_without_extensions, as well as
removing the "Extensions" column from the initial view of the action list
(though the user can add it back again using the column picker) and using the
file type description instead of extension as the primary field.

This won't solve the problem for anyone who has explicitly set
browser.download.hide_plugins_without_extensions in their user preferences file,
so the real solution is probably to change the code to ignore that setting.

Alternatively, one could come up with an alternative method for getting a list
of extensions, but presumably GNOME made a deliberate choice to remove this
functionality so there is presumably a good reason for it and it is probably
better to adapt firefox-for-unix to no longer rely on extensions. Of course,
firefox for windows will still need them, so any patch will have to be
unix-specific (or maybe even linux-specific) with appropriate #ifdef's if
pushed into the upstream firefox tree. I don't know enough about firefox
development to know what those #ifdefs should be, so have not attempted such a
patch.