It is not really that complex. All you do is add a record to one table in the database. The Admin Guide even gives you the syntax (slightly modified here for .mts):
Code:
INSERT INTO [gs_MimeType] ([FileExtension],[MimeTypeValue],[BrowserMimeTypeValue])
VALUES ('.mts','video/mts', '')
The Admin Guide has another section that talks about tools for editing database records: Management Tools for SQLite, SQL CE and SQL Server
Roger Martin
Creator and Lead Developer of Gallery Server Pro