TM upload issue

Dear all,

I received a TM in txt format. After creating a memory in Studio 2015, I chose upgrade TM in order to upload the client TM and when I chose to add a file based TM, I immediately received this error message after clicking on the TEXT CONVERSION box :

"The translation memory [directory path] could not be loaded: 'Invalid file format - no valid signature found'

And i couldn't upload the txt file.

What should be done?

Thanks in advance

Anne