Ensuring that a file uploaded via FormFunction is of a particular type

Question

I have a FormFunction that should work only with CSV files, something like:

CloudDeploy[FormFunction[{"file" -> "CSV"}, Dimensions[#file] & ], 
 Permissions -> "Public"]

How can I create a restriction so only CSV files can be submitted?

I want to show a message saying "only CSV files can be uploaded" when someone tries to load an image, and prevent the submit.

I tried this, didn't work:

CloudDeploy[
 FormFunction[{"file" -> <|"Interpreter" -> Restricted["CSV"]|>}, 
  Dimensions[#file] & ], Permissions -> "Public"] 

Answer 1

Instead of having FormFunction directly convert CSV input into a table of values, upload the CSV file to a CloudObject and do the CSV import in the cloud:

CloudDeploy[
   FormFunction[{"x" -> Interpreter[Restricted["UploadedFile", "CSV"]]},
      Dimensions[Import[#x, "CSV"]] & ], 
   FileNameJoin[{$CloudRootDirectory, "CSVRestrictedUploadedFile"}],
   Permissions -> "Public" ]

An attempt to submit a file other than type CSV will fail and generate an “Enter a file of type CSV” alert under the error condition input field:

See documentation "UploadedFile" (interpreter type).