Corpus Import Needs More Guardrails

[JSv4]

Accidentally imported a .pdf as a corpus instead of a document. This created a placeholder corpus and kicked off importer. The importer rightly detected the incorrect file type and threw and error, but a) I didn't get this error on the frontend and b) I know have a corpus I didn't expect.

Suggest the exports are given a custom extension and import is restricted to files with those extensions on the frontend.

[JSv4]

Suggest this be rolled into #140