closes #5641

src/controllers/uploads.js

@@ -209,7 +209,8 @@ uploadsController.uploadFile = function (uid, uploadedFile, callback) {
 	}
 
 	var allowed = file.allowedExtensions();
-	var extension = path.extname(uploadedFile.name);
+
+	var extension = path.extname(uploadedFile.name).toLowerCase();
 	if (!extension || extension === '.' || (allowed.length > 0 && allowed.indexOf(extension) === -1)) {
 		return callback(new Error('[[error:invalid-file-type, ' + allowed.join(', ') + ']]'));
 	}

src/file.js

@@ -81,7 +81,7 @@ file.allowedExtensions = function () {
 		if (!extension.startsWith('.')) {
 			extension = '.' + extension;
 		}
-		return extension;
+		return extension.toLowerCase();
 	});
 
 	if (allowedExtensions.indexOf('.jpg') !== -1 && allowedExtensions.indexOf('.jpeg') === -1) {