File extension QTL is used by the Quicktime media player from Apple, Inc. A QTL file (QuickTime Link) in its most common form is just an alternative extension for a Quicktime video or audio file. The reason the QTL file extension exists is simply to enable a movie posted on the internet to open in the Quicktime Player rather than the web browser plug-in.
When a user clicks a link to a File extension MOV file the web browser will load the file into the browser window for display with the plug-in, but when the user clicks a QTL file link, the file type is not registered to the plug-in, only the full player, and so the user will be prompted to download the file, or to open it with Quicktime Player. The QTL extension (MIME type application/x-quicktimeplayer) is recognised by Quicktime Player version 5 or greater, so you will need this software to open files of this type, although MOV and QTL are basically the same file type, so any MOV can be renamed with the QTL extension and any QTL file will work as a MOV file.
Since posting media on the internet in this way requires the user to download the whole movie before watching it and the movie is left on the user's hard disk, a lot of people choose to make the QTL file a small movie with a reference to the main movie. In this way the movie will still open in Quicktime Player but will play as it downloads, and only the reference QTL file will be left on the user's computer.
Another valid form of QTL file includes XML data containing the reference to the actual movie to be displayed. There have been a number of security vulnerabilities related to this type of QTL file that have allowed attackers to execute malicious programs on the viewer's computer. To be safe, it is wise to only open QTL file links on website that you trust.
For more general information about files, file extensions and registry you can read one of the following articles:
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