message: If a `show` mimetype is not recognised, then treat it as a download.
diff: @@ -2064,11 +2064,14 @@ export class EtTerminal extends ThemeableElementBase implements Commandable, Acc
  if (mimeType == null || isDownload) {
  this._appendMimeViewer("application/octet-stream", bulkFileHandle);
  } else {
  - this._appendMimeViewer(mimeType, bulkFileHandle);
  + const newViewer = this._appendMimeViewer(mimeType, bulkFileHandle);
  + if (newViewer == null) {
  + this._appendMimeViewer("application/octet-stream", bulkFileHandle);
  + }
  }
  }
  - private _appendMimeViewer(mimeType: string, bulkFileHandle: BulkFileHandle): void {
  + private _appendMimeViewer(mimeType: string, bulkFileHandle: BulkFileHandle): ViewerElement {
  const mimeViewerElement = this._createMimeViewer(mimeType, bulkFileHandle);
  if (mimeViewerElement !== null) {
  this._closeLastEmbeddedViewer("0");
  @@ -2081,6 +2084,7 @@ export class EtTerminal extends ThemeableElementBase implements Commandable, Acc
  this._enforceScrollbackSize(config.scrollbackMaxLines, config.scrollbackMaxFrames);
  }
  }
  + return mimeViewerElement;
  }
  private _createMimeViewer(mimeType: string, bulkFileHandle: BulkFileHandle): ViewerElement {
  