If you add the .MRC extension in libraryInfo/ Document
<add name="Image" title="Image Library" defaultExtenstions=".png, .jpg, .jpeg, .gif" metaKeys="Author, AlternateText, Extension, Name, Height, Width, Size, Category"></add>
<add name="Document" title="Document Library" defaultExtenstions=".doc, .docx, .rtf, .txt, .pdf, .ppt, .pps, .pptx, .html, .xls, .xlsx, .MRC
" metaKeys="Author, Description, Extension, Name, Size, Category"></add>
you should be able to upload this file to a library of document type and see it using Documents Manager.
To process the client request to download this file you should have handler mapping for .MRC, otherwise the IIS will not be able to process your request.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items