The XMLB file is mainly associated with the XMLBCUI Compiler/ DeCompiler IDE, which according to the NBA2kStuff website, is "a front end user interface to make the process of decompiling, compiling, and editing of XML source scripts in Activision's Marvel Ultimate Alliance and X-Men Legends 2 games easier."
XMLB files contain information that define actions and attributes that control the API engine of a game, and you can customise the game features with the XMLB script. Moreover, XMLBCUI Compiler/ DeCompiler IDE files can be identified with the file extensions .XMLB and .ENGB.
Opening the File
To open a XMLB file, just right-click on it, next, click on "Open with," then select the "Choose Default Program"option, and select the "XMLBCUI Compiler/ DeCompiler IDE" software. If Windows does not list the software, click on "Browse" to search for it.
If despite of having the XMLBCUI Compiler/ DeCompiler IDE software, your computer cannot recognise or open the file, it is recommended that you run a scan to search for common registry errors, as your computer's registry might be damaged.