If the subtitle file is some 3-10 MB large then is a DVD subtitle. Install Vobsub.
If is 20-100 Kb then try to open it with Wordpad for example. See if is a text file something like this:
0:0:10;05-->0:0:20;15
This is a subtile line
or:
{1200}{1300}This is a subtitle line.
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