MIME application/x-msdownload

Application/x-msdownload MIME type points to DLL and EXE files that are extensively used within the framework of Microsoft Windows operating system. Dynamic Linked Libraries or DLL is a file type that provides for executable data to be used exclusively by the Windows operating system. And EXE files are provided for users directly to execute programs within the operating system.

.dll file extensions are used my Microsoft Windows Operating system at its core to execute various functions. These are not directly executable by users, and are referred to by other applications developed for the Windows platform. DLL as well as EXE files can be carriers of malicious code too.

Most email services, for this reason, disallow direct email transmission of such files, with a view to plug spread of viruses through their service.

Use of MIME application/x-msdownload

The “x” before msdownload in the type name is an indicator that this MIME file extension is not covered by the definitions of the International Assigned Number Authority (IANA)

Despite the extension not being registered with IANA, it is extensively used by Microsoft Windows. Whenever there is a file uploaded through Internet Explorer browser of Microsoft, there is a process named “FindMIMEforData” which does a scan of the file and tags it with application/ x-msdownload tag.

On the other hand, while opening a message with the file enclosed, the browser reads the application/x-msdownlad extension and gives a popup to the user, prompting him to download the attachment file, rather than showing it inline within the mail.

To summarize, mime application/x-msdownload is an extension not defined within the standards mentioned by IANA. It is an extension that is developed and used by Microsoft Windows operating system and its bundled browser – Microsoft Internet Explorer. There are some mail services that block downloads of DLL and EXE files, which are prime suspects for spreading malicious code and viruses.

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