Sometimes the developers will ask for a small sample of your media to test on their end in order to troubleshoot a problem.  If the file size is too large to upload somewhere, then you may want to provide a smaller portion of that video which still exhibits the reported error for testing. MKVToolnix can input other extensions than MKV, but output will always be MKV and changing the extension may not produce the same issue. 1. Download and install MKVtoolnix Click the link below