[FIX] Make basic check of input filetype more robust to poor file naming

[alyssadai]

Closes #90

Changes proposed in this pull request:

• Check for apparent CSV filetype now looks at the suffix instead of anywhere in the filename for ".csv"

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[surchs]

looks good 🧑‍🍳

is it worth to add a (silly) test case that would not have passed before?

[alyssadai]

@surchs added a test for the file suffix check! (fc5e0ff)

want to have a quick look and, if it makes sense, check off the box in the checklist? 😄