The .undefined file extension is used for various file formats that cannot be recognized by the system. Some file recovery tools use this file extension to label recovered files that have not been completely restored. Other programs like Web browsers and email applications may label downloaded or attached files with unknown file types as UNDEFINED files.The content of these .undefined files varies, depending on the actual format of the files. For example, a data recovery application that has failed to completely restore a JPG file may label the file as an UNDEFINED file, and this .undefined file may contain corrupted image data. On the other hand, a Web browser that fails to recognize an AVI file that has been downloaded from the Internet may label the digital video file as an UNDEFINED file. This .undefined file may contain digital video, but there is no guarantee that the .undefined file will play in a standard AVI player.It's best to leave these .undefined files alone. In most instances, these UNDEFINED files contain corrupted data and are from unknown sources.
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If you have additional information about the UNDEFINED file format or software that uses files with the UNDEFINED suffix, please do get in touch - we would love hearing from you.