OSTOutlook Offline Storage File
OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from email server.
MHTMIME Encapsulation of Aggregate HTML
Files with MHT extension represent a web page archive format that can be created by a number of different applications. The format is known as archive format because it saves the web HTML code and associated resources in a single file. These resources include anything linked to the webpage such as images, applets, animations, audio files and so on.