File must supply character since the media type is text
One answer
When you upload files such as .xsl, .doc the application detects it as a non text file and sets the mime type to binary. Looks like it is not able to do so. Can you try out the following: Once you have selected the file and before save/upload it, go to the actions column and click on 'Edit file properties' action/button. You will now be able to set the mime type, line delimiter and encoding for the file. Set the appropriate values and then save/upload it.