You can do this using the command line, try:
$ vim -c "set fileformat=mac"
-c <command> is executed before loading the first file. As jammessan noted, this will only affect the first downloaded file, for subsequent files you will need to change the fileformat for each buffer.
If this does not work, you can also try:
$ vim -c "e ++ff=mac"
But it is better to configure this in your .vimrc on this machine:
set fileformats=mac,unix,dos
indicates which formats should be used when starting a new buffer. It may also affect the choice of fileformat for existing files.
See help file-formats :help 'fileformat' and :help 'fileformats' .
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