Sidecar files, also known as buddy files or connected files, are files which store data (often metadata) which is not supported by the source file format.
For each source file one or more sidecar files can be created. This is in contrast to "metadata databases" where one database contains metadata for several source files.
In most cases the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension. The problem with this system is that most operating systems and file managers have no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.
- Amiga Hunk metadata
- In AmigaOS, a file with a
.infoextension contains metadata for a companion Amiga Hunk executable file.
- Extensible Metadata Platform
- Extensible Metadata Platform (XMP) metadata is stored in a sidecar file when either a file format doesn't support embedded XMP metadata or if the workflow requires this.
- Connected Web Files and Folders
- A file system object that associated two or more files. The file system treats connected files are treated as a unit for purposes of moving, copying, and deleting. Some versions of Internet Explorer and Microsoft Word can save an HTML and its hyperlinked assets as such a unit.
- Many digital cameras will store a .thm (thumbnail) file alongside a recorded movie, with the same base filename as the movie file. These thumbnail files are JFIF-encoded image files. This system allows for quickly displaying a still preview of the movie, and storing camera data which is not supported by the AVI file format.
- JPEG + WAV
- Some digital cameras allow for voice/audio annotations with photos. These are then stored as WAV audio files alongside the JPEG photo file, with the same base filename.
- Mac OS resource fork
- The Mac OS operating system has internal support for metadata/resources that are not stored inside the file data. Because the support is built into the operating system, these resource forks will not show up as separate files, and all applications inherit support for resource forks. However, when files with a resource fork are copied over to a non-Mac OS disk format, such as an ISO-format CD-ROM or a MS-DOS compatible disk, this resource fork will be stored as a separate file alongside the main source file.
- Meta Information Encapsulation (MIE)
- Meta Information Encapsulation sidecar files. The MIE format is an extensible, dedicated meta information format part of ExifTool. MIE files can be used to encapsulate meta information from many sources and bundle it together with any type of file.
A variation of this are copies of the source file which contain largely the same information, but in a different format or from a previous version:
- Since many JPEG editing software used to destroy Exif metadata stored in digital photos, some photo cataloging applications allow to extract the Exif data and store that in an .exf file, so that the metadata can later be re-inserted into the JPEG file.
- Raw + JPEG
- Many digital cameras allow to store both uncompressed raw data and a JFIF-encoded image file when shooting in "raw mode". This allows for faster previewing the photo, and support by applications that do not support the (often undocumented) raw format.