When a user clicks Upload Multiple Files on Upload.aspx, the page for uploading documents, an <OBJECT> tag for the control is created on the page as follows:

<OBJECT id=idUploadCtl name=idUploadCtl

CLASSID=CLSID:07B06095-5687-4d13-9E32-12B4259C9813

WIDTH='100%' HEIGHT='350px'></OBJECT>

Note:
The CLSID used in this tag is the GUID that uniquely identifies the ActiveX control that is installed with the Microsoft Office system. To implement a custom control for uploading documents, specify a different GUID.

When the 2007 Microsoft Office system is installed on the client computer, this control is defined in the STSUPLD.DLL file, a dynamic-link library (DLL) that is installed on the client during 2007 Office System setup in the Local_Drive:\Program Files\Microsoft Office\OFFICE12 directory.