Introduce a list of image mime types we can support as images

E.g. TIFF will now be handled as a generic file, not image, even though it has mime prefix "image/"

src/models/attachmentManager.ts

@@ -77,7 +77,7 @@ export class AttachmentManager {
 
   private async prepareUpload(attachment: Attachment, room: KeanuRoom): Promise<Attachment> {
     const file = attachment.file;
-    if (file.type.startsWith("image/")) {
+    if (utils.isSupportedImageType(file.type)) {
       let url = URL.createObjectURL(file);
       attachment.src = url;
       if (attachment.src) {