This plugin does this:
https://github.com/spadgos/sublime-DefaultFileType
it seems pretty cool
Edit:
Well, two things, at present, there seems to be a small mistake, so the text file syntax is not selected correctly due to spaces in the file name. In addition, you need to set "use_current_file_syntax" to false (otherwise, the new file type will use the type of files that you opened when you pressed Ctrl-N) ... Thus, the fix / workaround is this:
Put the following code in: Packages / User / default_file_type.sublime-settings
{ "default_new_file_syntax": "Packages/Text/Plain_text.tmLanguage", "use_current_file_syntax": false }
NOTE.
Then find the file "Plain text.tmLanguage", copy and rename it (in the same folder) to "Plain_text.tmLanguage". [be sure to copy / duplicate it, not just rename it, as it may have dependencies]
Restart to be sure, and this should work. Also note that this plugin only works for new files created with Ctrl-N.
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