Improve format detection in Validata

• As a user,
• I want Validata to accept URLs targeting tabular files even if not containing an extension (e.g. .csv, .xls),
• in order to validate files from all sources.

Example

https://opendatafrance.opendatasoft.com/explore/dataset/digne-prenoms-2017/download/?timezone=Europe/Berlin&use_labels_for_header=true&format=csv

Acceptation criteria

• Validata, should accept URLs for CSV files not ending with .csv.
• Should accept .txt.
• Should accept .csv.txt.