FileInfo.com - The File Extensions Resource

.ALT File Extension

File Type	Dynamics AX Temporary File
File Format	File
Common?	No
File Description	Application label temporary file generated by Microsoft Dynamics AX; contains new labels created within program; saved as a standard label data (.ALD) file once the Dynamics AX Application Object Server has shut down.
Program(s) that open alt files	Windows: Microsoft Dynamics AX

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/alt