Relax document requirements for media gallery

ts/sql/Server.node.ts

@@ -5583,8 +5583,6 @@ function getSortedMedia(
     // see 'isFile' in ts/util/Attachment.std.ts
     contentFilter = sqlFragment`
       message_attachments.flags IS NOT ${VOICE_MESSAGE} AND
-      message_attachments.contentType IS NOT NULL AND
-      message_attachments.contentType IS NOT '' AND
       message_attachments.contentType IS NOT 'text/x-signal-plain' AND
       message_attachments.contentType NOT LIKE 'audio/%' AND
       message_attachments.contentType NOT LIKE 'image/%' AND

ts/util/Attachment.std.ts

@@ -542,10 +542,6 @@ export const isVisualMedia = (attachment: AttachmentType): boolean => {
 export const isFile = (attachment: AttachmentType): boolean => {
   const { contentType } = attachment;
 
-  if (isUndefined(contentType)) {
-    return false;
-  }
-
   if (isVisualMedia(attachment)) {
     return false;
   }