Helix Server file types determine how a file is stored in the depot and synced to the workspace, and whether it can be diffed.
To change a file’s Helix Server file type (or other storage attributes):
Context-click the desired file and choose Check Out.
The file is checked out.
Context-click the file and choose Change Filetype….
The Change Filetype dialog is displayed.
For details about file types and attributes, see "File Types" the in the P4 Command Reference.