Our standard media library accepts both images (GIF, JPG, PNG) and "office" files such as .txt, .rtf, .xlsx, etc.
To prevent the spread of malware the library does not accept file extensions it does not recognize.
If you need to extend the set of accepted file extensions, you can do so in an afterInit callback in your app.js configuration. Here is code to add a file extension to the office group.
afterInit: function(callback) {
var office = _.find(site.apos.fileGroups, function(group) {
return group.name === 'office';
});
office.extensions.push('csv');
office.extensions.push('tsv');
return callback(null);
}
Note: adding new file extensions to the images group does not make sense, since web browsers still won't support them in the img tag, in style attributes, etc.