** This is tested in ODOO V15 **

if you are using FileReader() in JavaScript before sending the file, 
the FileReader is appending the “data URI scheme” which makes it impossible for odoo to identify the file, as a result “application/octet-stream” is assigned to it when it's saved in ir_attatchement table,

the solution is to remove this prefix so that odoo can decode the base64 encoded file correctly and assign it the correct mimetype in ir_attatchement

    handleFileChange(event) {
      const file = event.target.files[0]; // Get the selected file
      if (file) {
        // If a file is selected, we need to read it (e.g., as base64 or binary)
        const reader = new FileReader();
        reader.onloadend = () => {
          // Update the state using the setter function provided by useState
          // the string below is removed so that odoo can recognize mimetype of pdf files
          this.state.file = reader.result.replace(/^data:.+;base64,/, "");
        };
        reader.readAsDataURL(file); // Read the file as a data URL (base64 encoded)
      } else {
        this.state.file = null; // Clear the preview if no file is selected
      }
    }