I think this reply re source-type was voted Answer because the OP's problem was with source-type rather than the spaces in the file-name/path
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My experience with Splunk v7.3 is that the [monitor] stanza understands 'spaces' as-is, i.e., nothing special needs to be done.
For instance, my 'inputs.conf' works correctly with the following:
[monitor://C:\Program Files (x86)\My App\Logs*\app.log*]
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