We have tried with the .csv dataset, we are able to successfully create the dataset in ml.azure.com for both types i.e File (uri_file) and MLTable.
Note: For MLTable, put your .csv file inside a special folder created for this purpose and it would allow you to SELECT that folder.
Please find the snapshot for the successful upload in both cases. If possible please share the csv file to debug on this.
I hope you understand!