api: iso up/download: check file content

by letting it run through 'file_size_info' as 'untrusted', since that
does the necessary checks. We do this so we don't accidentally
up/download a file that is not a valid iso

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>

src/PVE/Storage.pm

@@ -2196,4 +2196,14 @@ sub get_import_metadata {
     return $plugin->get_import_metadata($scfg, $volname, $storeid);
 }
 
+# dies if the content of the given path is unexpected for an ISO
+sub assert_iso_content {
+    my ($path) = @_;
+
+    # check for things like backing image
+    file_size_info($path, undef, 1);
+
+    return 1;
+}
+
 1;