Generic extension applied to a file lacking a valid extension
FILE is a generic file extension automatically applied to a file in Windows that lacks a valid extension. The FILE file's extension may be unknown or it may actually lack an extension.
Serdar Yegulalp explains: "Inaccurate or missing extensions can occur if a file has been munged during a disk recovery or automated file-renaming process or through simple user error. To determine the file type for such a file, you could open the file in a binary / hex editor and use your best guess after examining the content."
To determine a FILE file's extension, you can look at the file itself or contact the source to ask what kind of file it is. If you can identify the program associated with the file, determine the default file format for that program and rename the file with that file extension and then try to open it.
Alternatively, you can try a file identifier program, such as TrID or TrIDNet, that will match a file against known formats.
Learn more about FILE files:
Serdar Yegulalp explains using TrID to identify files without valid extensions.
IT Knowledge Exchange maintains FAQs about file extensions.