You are correct.  It appears as though they meant to change the scope to no longer allow file extensions to be set in the Gallery configuration.  The issue is also with lblMediaExtensions.  Until Core Team updates the download package, here is a fix I applied that seems to be working fine without having to recompile:

I changed lines 82-90 in Settings.ascx to (changes highlighted):

<tr>
 <td class="Gallery_RowPanel" width="220" height="24">&nbsp;
  <dnn:label id="plImageExtensions" runat="server" text="Image Extensions:" controlname="lblImageExtensions"></dnn:label></td>
 <td class="Gallery_Row" align="left"><asp:label id="lblImageExtensions" runat="server" CssClass="Normal"></asp:label></td>
</tr>
<tr>
 <td class="Gallery_RowPanel" width="220" height="24">&nbsp;<dnn:label id="plMediaExtensions" runat="server" text="Media Extensions:" controlname="lblMediaExtensions"></dnn:label></td>
 <td class="Gallery_Row" align="left"><asp:label id="lblMediaExtensions" runat="server" CssClass="Normal"></asp:label></td>
</tr>

Hope this helps others experiencing this issue.

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~DJ Doug
www.pricecorp.com

Daniele, I received that error prior to making the changes.  After I made the changes, I no longer get that error and am able to access the configuration page with no problems.  Try to cut and paste my changes into Notepad (to remove the html formatting from these forums), then cut/paste from Notepad into Settings.ascx over the 2 rows that use Textbox for Media and Image extensions (Lines 82-90 in Visual Studio).  (Be sure to backup your files first!)  Save Settings.ascx, browse to Gallery, hit Refresh, then try to access configuration.  If you are still receiving the error, can you please send the full error details from the Log Viewer (under Admin tab).  Hope that helps.

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~DJ Doug
www.pricecorp.com