You can't access the media resources directly because the files are stored in a virtual environment called SDCard.FS file. To mount the file, download OSFMount application and and then locate the SDCard.fs file on
C:\ProgramData\Bluestacks\Android\SDCard.fs. Open OSFMount and select Mount New and untick the Read Only. Make sure the mounted image is set to auto for Driver. You can use My Computer afterwards to browse the SDCard.