What Is a Sidecar File?
A sidecar file is a separate companion file that stores information associated with another file. In image and video workflows, it can hold descriptive metadata, editing instructions, captions, or other supporting data without placing that information inside the original media file.
A common example is a RAW photograph accompanied by an XMP sidecar. It can preserve image adjustments and metadata while leaving the source image unchanged. The sidecar supports the original asset; not replacing it.
For developers and media teams, sidecars make it possible to manage an asset’s supporting information separately. That flexibility also creates a responsibility: the relationship between the files must survive uploads, renaming, transfers, and backups.
How a Sidecar File Works
An application creates or updates a sidecar when it needs to save information outside the primary file. When reopening the asset, compatible software reads the companion file and applies or displays the relevant information.
Many applications associate files through their names and locations. For example, a RAW photograph named chair-hero.nef might have an accompanying chair-hero.xmp. Other applications use different conventions, including retaining the original extension within the sidecar’s filename.
In an automated workflow, teams can also record the relationship explicitly through asset identifiers or a manifest. This avoids relying entirely on filenames when files move between systems.
Common Uses of Sidecar Files
Sidecar describes a file’s relationship to another asset, rather than a single file format.
| Sidecar example | Supporting information | Typical purpose |
|---|---|---|
| XMP beside a photograph | Keywords, ratings, copyright information, or supported editing settings | Metadata exchange and image editing |
| WebVTT beside a video | Timed captions, subtitles, or chapter information | Accessible and localized playback |
| Custom JSON beside a media asset | Fields defined by the production workflow | Application-specific processing or asset exchange |
For web video, a WebVTT file is associated with the media through a player configuration or an HTML text track. Multiple language tracks can also accompany the same video, too. An XMP sidecar containing image adjustments works differently: compatible editing software must interpret those instructions to render the intended appearance.
Sidecar Files vs. Embedded Metadata
Embedded metadata lives inside the media file. Sidecar metadata lives outside it.
Neither approach applies universally. In Lightroom Classic, metadata for proprietary RAW formats is written to external XMP sidecars, while formats such as JPEG, TIFF, and DNG can carry XMP internally. The application also maintains information in its catalog.
For a DAM ingestion workflow, this distinction affects what must be transferred. Uploading only the photograph may omit information stored externally. Uploading both files still requires the receiving platform to recognize their relationship and support the relevant fields.
Why Sidecar Files Matter
Preserving the relationship between media and sidecar files helps retain useful production information. However, a website delivery workflow should receive rendered images with the approved appearance. Just having an XMP file alongside a picture doesn’t ensure that the adjustments will be applied by a browser or transformation service.
Teams should therefore distinguish between preserving editable source material and preparing assets for publication.
