Problem: you try to deploy your xslt file from Visual Studio, and get an error saying "Value for the parameter 'MimeType' is not specified. it is either missing from the function call, or it is set to null".

Solution: In Solution Explorer, click on the xsl file. Then switch to the Properties pane and enter "text/xml" as the MimeType. Note: this is not required when deploying the xsl through Report Manager's upload button.