Fix bug: requests with a 'file' argument work now

[artoonie]

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PR Summary

e.g. Fix bug for requests with a 'file' argument. The current code assumes 'file' is always a file's content (upload), whereas there are many requests that take a fileID with the argument name 'file'.

Related Issues

N/A (that I have found)

Test strategy

e.g. Test files.info in addition to the files.upload test that already exists.

[coveralls]

Coverage increased (+0.06%) to 61.328% when pulling ea6f490 on artoonie:bugfix/file-info into 7dfa7f3 on slackhq:master.

[coveralls]

Coverage increased (+0.5%) to 61.719% when pulling ea6f490 on artoonie:bugfix/file-info into 7dfa7f3 on slackhq:master.

[artoonie]

Not sure why coveralls is failing on the py35 build...is this normal? Locally, I get the same issue on master.

[jammons]

This looks good. Thanks!

I just retriggered the test run and it passed (I don't know why that happens, but it does sometimes).