Q:

SPFx uploading and adding attachment to a list

I am having some difficulty upload and attachment to a list item in sharepoint using the PNP/SP package. I dont have much experience with the input file component so I think I may be missing a step between the file upload html element and submitting the file to the SharePoint web service.
So far I've tried to follewing the PNP example with a few changes https://pnp.github.io/pnpjs/sp/docs/attachments/ and tried a few different arguments but they all tend to result in 409 or 500 errors, one error mentions that it's attempting a GET request instead of post.
My code is below and i'll post full error messages when i get into the office tomorrow but any help would be greatly appreciated.
private setButtonsEventHandlers(): void {   
let fileUpload = document.getElementById("fileUploadInput") 
if(fileUpload) {
  fileUpload.addEventListener('change', () => { 
    this.uploadFiles(fileUpload); 
  });
}
  }

private async uploadFiles(fileUpload) {
  let file = fileUpload.files[0];
  let attachmentsArray = this.state.attachmentsToUpload;

  let _web = new Web(this.props.wpContext.pageContext.site.absoluteUrl);
  let _listItem;
  let listUrlSplit: string[] = this.props.listUrl.split("/");
let listName: string = listUrlSplit[listUrlSplit.length-1];
  _listItem = await     _web.lists.getByTitle(listName).items.getById(this.props.id);

  let attachmentUpload = await _listItem.attachmentFiles.add(file.name,file)
}

I tested the code (below) by replacing my file upload with strings and it does work so I think my error is in misunderstanding the input file element
let attachmentUpload = await _listItem.attachmentFiles.add("Testfile.txt","This is test content")

Thanks in advance all and enjoy whats left of sunday ;)
Andy

A:

Here is my simple test demo which works(React framework).
Component .tsx
<div className={ styles.column }>
              <input type='file' id='fileUploadInput' name='myfile'/>
              <span className={ styles.title }>Welcome to SharePoint!</span>
              <p className={ styles.subTitle }>Customize SharePoint experiences using Web Parts.</p>
              <p className={ styles.description }>{escape(this.props.description)}</p>
              <a href="https://aka.ms/spfx" className={ styles.button }>
                <span className={ styles.label }>Learn more</span>
              </a>
            </div>

webpart.ts
public render(): void {
    const element: React.ReactElement<IPnpspUploadAttachementProps > = React.createElement(
      PnpspUploadAttachement,
      {
        description: this.properties.description
      }
    );

    ReactDom.render(element, this.domElement);
    this.setButtonsEventHandlers();
  }

  private setButtonsEventHandlers(): void {   
    let fileUpload = document.getElementById("fileUploadInput") 
    if(fileUpload) {
      fileUpload.addEventListener('change', () => { 
        this.uploadFiles(fileUpload); 
      });
    }
    }

    private async uploadFiles(fileUpload) {
      let file = fileUpload.files[0];
      //let attachmentsArray = this.state.attachmentsToUpload;        
      let item = sp.web.lists.getByTitle("MyList").items.getById(15);
      item.attachmentFiles.add(file.name,file).then(v => {
          console.log(v);
      });
      //let attachmentUpload = await _listItem.attachmentFiles.add(file.name,file)
    }

