[KevinH]

In python you can import imghdr to find out what type of image a data file is and then use a file type extension lookup in a dictionary (hash table)

Here is an piece of the OPFResource.cpp file:

// Initializes m_Mimetypes
void OPFResource::CreateMimetypes()
{
    m_Mimetypes[ "jpg"   ] = "image/jpeg";
    m_Mimetypes[ "jpeg"  ] = "image/jpeg";
    m_Mimetypes[ "png"   ] = "image/png";
    m_Mimetypes[ "gif"   ] = "image/gif";
    m_Mimetypes[ "tif"   ] = "image/tiff";
    m_Mimetypes[ "tiff"  ] = "image/tiff";
    m_Mimetypes[ "bm"    ] = "image/bmp";
    m_Mimetypes[ "bmp"   ] = "image/bmp";
    m_Mimetypes[ "svg"   ] = "image/svg+xml";
    m_Mimetypes[ "ncx"   ] = NCX_MIMETYPE;
    // We convert all HTML document types to XHTML
    m_Mimetypes[ "xml"   ] = "application/xhtml+xml";
    // m_Mimetypes[ "xml"   ] = "application/oebs-page-map+xml";
    m_Mimetypes[ "xhtml" ] = "application/xhtml+xml";
    m_Mimetypes[ "html"  ] = "application/xhtml+xml";
    m_Mimetypes[ "htm"   ] = "application/xhtml+xml";
    m_Mimetypes[ "css"   ] = "text/css";
    m_Mimetypes[ "mp3"   ] = "audio/mpeg";
    m_Mimetypes[ "oga"   ] = "audio/ogg";
    m_Mimetypes[ "ogg"   ] = "audio/ogg";
    m_Mimetypes[ "mp4"   ] = "video/mp4";
    m_Mimetypes[ "ogv"   ] = "video/ogg";
    m_Mimetypes[ "webm"  ] = "video/webm";
    m_Mimetypes[ "smil"  ] = "application/smil+xml";
    m_Mimetypes[ "pls"   ] = "application/pls+xml";
    m_Mimetypes[ "js"    ] = "text/javascript";
    // Until the standards gods grace us with font mimetypes,
    // these will have to do
    m_Mimetypes[ "otf"   ] = "application/vnd.ms-opentype";
    m_Mimetypes[ "ttf"   ] = "application/x-font-ttf";
    m_Mimetypes[ "ttc"   ] = "application/x-font-truetype-collection";
    m_Mimetypes[ "woff"  ] = "application/font-woff";
}

Here is a similar piece of python code used by the launcher:

ext_mime_map = {
                '.jpg'  : 'image/jpeg',
                '.jpeg' : 'image/jpeg',
                '.png'  : 'image/png',
                '.gif'  : 'image/gif',
                '.svg'  : 'image/svg+xml',
                '.xhtml': 'application/xhtml+xml',
                '.html' : 'application/xhtml+xml',
                '.ttf'  : 'application/x-font-ttf',
                '.otf'  : 'application/x-font-opentype',
                '.woff' : 'application/font-woff',
                '.mp3'  : 'audio/mpeg',
                '.mp4'  : 'video/mp4',
                '.css'  : 'text/css',
                '.ncx'  : 'application/x-dtbncx+xml',
                '.xml'  : 'application/oebs-page-map+xml',
                '.opf'  : 'application/oebps-package+xml',
                '.smil' : 'application/smil+xml',
                '.pls'  : 'application/pls-xml',
                '.js'   : 'text/javascript',
                '.epub' : 'application/epub+zip',
                #'.js'   : 'application/javascript',
                #'.otf'  : 'application/vnd.ms-opentype',
                }

You can find exmples of how to use imghdr in the KindleUnpack mobi_cover.py link I sent to you. That piece of python code does the same thing.