Opening UNDEFINED files

Have a problem opening a .UNDEFINED file? We collect information about file formats and can explain what UNDEFINED files are. Additionally we recommend software suitable for opening or converting such files.

What is the .UNDEFINED file type?

All occurrences of the .undefined extension have to do with software bugs and possibly improper configuration. The .undefined extension itself, when assigned to a file, is an indication of a program's failure to recognize a particular file format and/or assign a proper extension.

The .undefined extension could be appended to media files like MP3 (instead of the proper .mp3 in this case) downloaded from some online FFMPEG-based audio/video conversion services, due to improper configuration or software bugs. If the target file format/type is known, one may attempt to manually change the extension (i.e., .undefined to .mp3).

It can also be a result of erroneous MIME-type handling by a web server, i.e. the server does not know for some reasons what MIME type should be used for any specific content served, so the .undefined extension is assigned.

An error message about an undefined extension could also appear on certain Joomla-based web sites when such a site was accessed with one of earlier versions of Mozilla Firefox and an attempt was made to upload files (incl. images, etc.). It was caused by a software bug in Firefox that was corrected in later versions.

Software to open or convert UNDEFINED files

You can open UNDEFINED files with the following programs:
VLC media player
VLC media player by VideoLAN
Mozilla Firefox
Mozilla Firefox by Mozilla
GOM Player
GOM Player by Gretech Corporation

Popular formats