Thumbnails, and why some accounts show plain icons

When you browse the Storage Studio location, Finder can show small previews of your photos, videos and documents, just like it does for files on your Mac. Storage Studio draws them by reading only a small part of each file, and only for the files Finder is about to show. On storage that charges you for every request, it shows plain icons instead, so browsing a folder never costs you money.

Which accounts show thumbnails

The rule is simple: thumbnails are drawn only where reading part of a file is free. Anything else gets plain icons, even if that means fewer pictures. Plain icons don't affect your files in any way; open a file and it downloads and opens as usual.

What gets a thumbnail

• Photos and pictures, including common camera raw formats. Most cameras and phones store a small preview near the start of the file, and Storage Studio reads just that part. A small picture with no built-in preview is read whole.
• Videos from phones and cameras. Storage Studio reads only the start and the end of the file, which is enough for the first frame, instead of downloading the whole video.
• Documents, such as PDFs, Word, Excel and PowerPoint files, Pages, Numbers and Keynote files, text and web pages. Small ones are read whole; for a large PDF only the parts needed for the first page are read. If that isn't enough, the plain icon stays.

%% caption: How Storage Studio decides whether to draw a thumbnail
flowchart TD
  A[Finder is about to show a file] --> B{Is the account free to read from?}
  B -->|No| P[Plain icon]
  B -->|Yes| C{Picture, video or document?}
  C -->|No| P
  C -->|Yes| D[Read only the small part needed]
  D --> E{Could a preview be drawn?}
  E -->|Yes| T[Thumbnail]
  E -->|No| P

Gentle by design

• Thumbnails are made only for files on screen. Scroll, and the next ones are fetched as they come into view.
• Only a few are fetched at a time, to stay light on your Mac and on the account.
• Any temporary file used to draw a thumbnail is deleted straight away. Drawing a thumbnail doesn't leave a downloaded copy of the file on your Mac.

Why not just show everything?

Some storage, including S3-compatible services, charges for every request, however small. Scrolling once through a folder of photos would mean a billed request for every icon. Storage Studio would rather show a plain icon than run up a bill you didn't ask for.

See also: Large folders appear a page at a time · S3-compatible storage · Cloud accounts