Automatically check if uploaded files are avro or not.

[hfjn]

What happens if I forget to set this flag but the data is avro? Wouldn't it be possible to detect avro or not automatically based on the first few lines?

Originally posted by @hfjn in #39

[MrTrustworthy]

Avro files have a header, see https://avro.apache.org/docs/1.8.1/spec.html#Object+Container+Files
I think in pyconnect we read something from the avro message struct to make sure it's avro or something... maybe check there too