Look in the Preferences > Editor > File Types under Text files, the name of your file is probably there, remove it.
If you cannot find it there, the next option is to search IntelliJ’s caches for the name of the file, e.g., find . -name "*.xml" | xargs grep "Whoopsies". IntelliJ has to be remembering that value somehow…
The cache locations depends on your operating system and IntelliJ version (source).
Windows
Windows Vista, 7, 8:
\Users\[USER ACCOUNT NAME]\.[PRODUCT][VERSION]
Windows XP:
[SYSTEM DRIVE]\Documents and Settings\[USER ACCOUNT NAME]\.[PRODUCT][VERSION]
*nix
~/.[PRODUCT][VERSION]
Mac OS X
Configuration:
~/Library/Preferences/[PRODUCT][VERSION]
Caches:
~/Library/Caches/[PRODUCT][VERSION]
Plugins:
~/Library/Application Support/[PRODUCT][VERSION]
Logs:
~/Library/Logs/[PRODUCT][VERSION]