Corrupt file when using Azure Functions External File binding - encoding

Question

I'm running a very simple ExternalFileTrigger scenario in Azure Functions were I copy one created image file from one onedrive directory to another.
function.json
{
"bindings": [
{
"type": "apiHubFileTrigger",
"name": "input",
"direction": "in",
"path": "Bilder/Org/{name}",
"connection": "onedrive_ONEDRIVE"
},
{
"type": "apiHubFile",
"name": "$return",
"direction": "out",
"path": "Bilder/Minimized/{name}",
"connection": "onedrive_ONEDRIVE"
}
],
"disabled": false
}
run.csx
using System;
public static string Run(string input, string name, TraceWriter log)
{
log.Info($"C# File trigger function processed: {name}");
return input;
}
Every things seems to work well BUT the new output image file i corrupt. The size is almost twice as big.
When looking at the encoding the original file is in ANSI but the new generated file from Azure Functions is in UTF-8.
It's working fine when I'm using a text file when source encoding is UTF-8.
Is it possible to force Azure binding ExternalFileTrigger to use ANSI? Or how to solve this?

Answer 1

UPDATE 2019: The external file binding seems to be deprecated from the current version of Azure Functions.
If you want to copy the file as-is, or do more fine-grained binary operations on the file contents, I recommend using Stream type instead of string for your input and output bindings:
public static async Task Run(Stream input, Stream output, string name, TraceWriter log)
{
using (MemoryStream ms = new MemoryStream())
{
input.CopyTo(ms);
var byteArray = ms.ToArray();
await output.WriteAsync(byteArray, 0, byteArray.Length);
}
log.Info($"C# File trigger function processed: {name}");
}
Change the ouput binding in function.json:
"name": "output",
This function will do exact binary copy of the file, without conversion.
You can see which other types you can use for bindings in External File bindings (see "usage" sections).