In Perforce, you can change the filetype of files stored in the depot either by:
The new filetype takes effect when you submit your opened files to the depot.
Selecting Change filetype... from the pulldown menu in the Files tab brings up a form that lets you enter a new filetype and reopen your opened files using that filetype. See Filetypes for a list of Perforce filetypes.
If you chose Change filetype... with a directory selected, the form allows you to select the files on which the Reopen command will operate. See Restricting Files Used in Commands for more information.
Click the "Reopen" button in the "Change Filetype" form after you have selected files and entered a new filetype. Note that this command only affects files that are already opened in your workspace.