Some non-image blobs can be previewed: that is, they can be presented as images. A video blob can be previewed by extracting its first frame, and a PDF blob can be previewed by extracting its first page.
A previewer extracts a preview image from a blob. Active Storage provides previewers for videos and PDFs: ActiveStorage::Previewer::VideoPreviewer and ActiveStorage::Previewer::PDFPreviewer. Build custom previewers by subclassing ActiveStorage::Previewer and implementing the requisite methods. Consult the ActiveStorage::Previewer documentation for more details on what's required of previewers.
To choose the previewer for a blob, Active Storage calls
accept? on each registered previewer in order. It uses the
first previewer for which
accept? returns true when given the
blob. In a Rails application, add or remove previewers by manipulating
Rails.application.config.active_storage.previewers in an
Rails.application.config.active_storage.previewers # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ] # Add a custom previewer for Microsoft Office documents: Rails.application.config.active_storage.previewers << DOCXPreviewer # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer, DOCXPreviewer ]
Outside of a Rails application, modify
The built-in previewers rely on third-party system libraries. Specifically, the built-in video previewer requires ffmpeg. Two PDF previewers are provided: one requires Poppler, and the other requires mupdf (version 1.8 or newer). To preview PDFs, install either Poppler or mupdf.
These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you install and use third-party software, make sure you understand the licensing implications of doing so.