How can I change the standard "save as" filetype of Notepad++?

I think you want to change the default file format for new documents that Notepad++ uses when you run the program from the icon. If that's correct. You can change the File format of a new document like this.

1. Navigate to C:\Program Files (x86)\Notepad++

2. Then open the file config.model.xml (if you use Notepad++ to edit you won't be able to save as the file is in use.)

3. Look for this line around line #28 <GUIConfig name="NewDocDefaultSettings" format="0" encoding="0" lang="1" />

4. Then you have to change the value for "lang=0" by default it opens everything in txt format. If you look at step 3 I have "1" because I use it for PHP. You'll have to check the source code if you want to change it to anything else. As the developers did not list all the languages in this configuration file you could use.

Post back if you need further explanation.

The list for file associations, picked up from the source code. Just change the set lang="langID"

    <associationMap>
    <!-- langID:
        L_TEXT: 0     L_PHP: 1        L_C: 2        L_CPP: 3       L_CS: 4         L_OBJC: 5
        L_JAVA: 6     L_RC: 7         L_HTML: 8     L_XML: 9       L_MAKEFILE: 10  L_PASCAL: 11
        L_BATCH:12    L_INI: 13       L_ASCII: 14   L_USER: 15     L_ASP: 16       L_SQL: 17
        L_VB: 18      L_JS: 19        L_CSS: 20     L_PERL: 21     L_PYTHON: 22    L_LUA: 23
        L_TEX: 24     L_FORTRAN: 25   L_BASH: 26    L_FLASH: 27    L_NSIS: 28      L_TCL: 29
        L_LISP: 30    L_SCHEME: 31    L_ASM: 32     L_DIFF: 33     L_PROPS: 34     L_PS: 35
        L_RUBY: 36    L_SMALLTALK:37  L_VHDL: 38    L_KIX: 39      L_AU3: 40       L_CAML: 41
        L_ADA: 42     L_VERILOG: 43   L_MATLAB: 44  L_HASKELL: 45  L_INNO: 46      L_SEARCHRESULT: 47
        L_CMAKE: 48   L_YAML: 49      L_COBOL 50    L_GUI4CLI: 51  L_D: 52         L_POWERSHELL: 53
        L_R: 54       L_JSP: 55
    -->
        <association langID="1" id="php_function"/>
        <association langID="2" id="c_function"/>
        <association langID="3" id="c_cpp_function"/>
        <association langID="6" id="java"/>
        <association langID="9" id="xml_node"/>
        <association langID="12" id="batch_label"/>
        <association langID="13" id="ini_section"/>
        <association langID="19" id="js_function"/>
        <association langID="21" id="perl_function"/>
        <association langID="26" id="bash_function"/>
        <association langID="28" id="nsis_syntax"/>
    <!--
        if langID cannot be found above, you can still set the file extensions
        <association ext=".my_passer_ext1" id="my_passer_id"/>
        <association ext=".my_passer_ext2" id="my_passer_id"/>

        for User Defined Languages:
        <association userDefinedLangName="my user defined language" id="my_udl_passer_id"/>
        <association userDefinedLangName="Autocad" id="my_autocad_passer_id"/>
    -->

    </associationMap>