Dealing with encoding in Flask file uploads/downloads

I have a react client that takes in user input as a file and sends it to my remote Flask server for storage. I send the file in the form of a Werkzeug FileStorage object and in the remote server I store it with file.save(path). In the react client I'm trying to build a way to download the file from the server, however I'm running into problems. Currently my program is working for downloading .txt files. I'm able to do this though a fetch javascript request:

 fetch(FETCH_URL, {
  method: 'POST',
  body: data,
  headers: {
    'Content-Type': 'application/json'
  }
}).then((response) => {
    var a = response.body.getReader();
    a.read().then(({ done, value }) => {
        saveAsFile(new TextDecoder("utf-8").decode(value), 'filename.txt');
      }
    );
});


function saveAsFile(text, filename) {
  const type = 'application/text'; // modify or get it from response
  const blob = new Blob([text], {type});
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click(); 
}

Thanks to some help I got in this post: Download file in react client from flask remote server

I know this code is specifically made to work only with .txt files based on the type being passed in to Blob, but the front end is not the real problem.

The real problem is in my remote flask server, the following code is what is called in the flask server:

 with open(filename, 'r') as f:
     contents = f.read()
     return contents

I tried returning the file itself but the server gives an error:

"ValueError: I/O operation on closed file." 

So I decided to return the contents of the file as shown above.

The problem arises when I try to get a file for example "download.jpeg". Reading the file gives the following error:

"UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte" 

From what I understand Flask works exclusively with 'utf-8' and I assume this means the file in the server is on 'utf-8' encoded.

Does anyone have a suggestion or guidance on a solution or a workaround maybe a way to change the files encoding when I save it on the server or something else that could help me with what I'm trying to do?

Answers:

Answer

Fetch's Response has blob() to convert the response directly to blob, so you don't have to read the stream, you don't have to find out it's content type or anything. Just try the below solution.

fetch(FETCH_URL, {
  method: 'POST',
  body: data,
  headers: {
    'Content-Type': 'application/json'
  }
}).then((response) => {
      response.blob().then((blob) => {
         saveBlob(blob, 'filename');
      });
});


function saveBlob(blob, filename) {
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click(); 
}

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