file names with 16 characters currently treated as UUIDs in insert tags

File names within a file or image insert tag that happen to be 16 characters long are currently treated as a UUID and the corresponding file is not found. This is because ```\Validator::isUuid``` always returns true for 16 character data, because it auto-converts from a binary to a text UUID.

This patch adds a new function ```isTextUuid```, which only accepts textual UUIDs, and uses this function where the insert tags are processed.