.plugin files are ASCII text files with a specific format:

Line 1: Plugin type. Currently the only supported value is PluginType=Export

Line 2: The text that will appear in the export dialog. This is in standard Windows format: [text]|[extension], where [text] is the text that will appear in the dropdown and [extension] is the filter to apply to the dialog view (i.e., *.doc).

Line 3: The executable to call, for example, export_html.exe. Since the .fsx file's name is just appended to the end of this line when the executable is called, you can add extra command-line options after the end of the executable name and read the final parameter as the .fsx file name. For example, the HTML export does this, since the same executable is used with different command line options for both the HTML and iPhone exports.

PluginType=Export
HTML webpage (*.html)|*.html
export_html.exe html