Fix extension in ds_fs_5tt_hsvs

[tsalo]

Closes #162.

Changes proposed in this pull request

• Specify extension in ds_fs_5tt_hsvs because it was defaulting to an invalid extension (.long_5tt.mif.gz).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 33.95%. Comparing base (df45b9d) to head (67bd52b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #165 +/- ## ======================================= Coverage 33.95% 33.95% ======================================= Files 57 57 Lines 7192 7192 Branches 983 983 ======================================= Hits 2442 2442 Misses 4652 4652 Partials 98 98 ```

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