diff --git a/api/src/CalDAV/JsBase.php b/api/src/CalDAV/JsBase.php
index 453f66e918..827a09c82a 100644
--- a/api/src/CalDAV/JsBase.php
+++ b/api/src/CalDAV/JsBase.php
@@ -370,6 +370,11 @@ class JsBase
 					$target = $value;
 				}
 			}
+			// if we unset fields stored directly in the database, they will NOT be updated :(
+			elseif (!$n)
+			{
+				$target = null;
+			}
 			else
 			{
 				unset($parent[$part]);
diff --git a/api/src/MimeMagic.php b/api/src/MimeMagic.php
index aaa93aafb2..9f8e0d6a56 100644
--- a/api/src/MimeMagic.php
+++ b/api/src/MimeMagic.php
@@ -115,6 +115,25 @@ class MimeMagic
 		return $key;
 	}
 
+	/**
+	 * Convert a MIME type to all known file extensions
+	 *
+	 * If we cannot map the type to any file extension, we return an empty array.
+	 *
+	 * @param string $_type  The MIME type to be mapped to a file extension.
+	 * @return array with possible file extensions
+	 */
+	public static function mime2extensions($_type)
+	{
+		$type = strtolower($_type);
+		if (isset(self::$mime_alias_map[$type])) $type = self::$mime_alias_map[$type];
+
+		return array_keys(array_filter(self::$mime_extension_map, static function($mime) use ($type)
+		{
+			return $mime == $type;
+		}));
+	}
+
 	/**
 	 * Fix old / aliased mime-types by returning valid/default mime-type
 	 *