Angular File Upload

Very easy and fastest method is using ng2-file-upload.

Install ng2-file-upload via npm. npm i ng2-file-upload --save

At first import module in your module.

import { FileUploadModule } from 'ng2-file-upload';

Add it to [imports] under @NgModule:
imports: [ ... FileUploadModule, ... ]

Markup:

<input ng2FileSelect type="file" accept=".xml" [uploader]="uploader"/>

In your commponent ts:

import { FileUploader } from 'ng2-file-upload';
...
uploader: FileUploader = new FileUploader({ url: "api/your_upload", removeAfterUpload: false, autoUpload: true });

It`is simplest usage of this. To know all power of this see demo

This way I implement upload file to web API in project.

I share for whom concern.

const formData: FormData = new FormData();
formData.append('Image', image, image.name);
formData.append('ComponentId', componentId);
return this.http.post('/api/dashboard/UploadImage', formData);

Step by step

ASP.NET Web API

[HttpPost]
[Route("api/dashboard/UploadImage")]
public HttpResponseMessage UploadImage() 
{
    string imageName = null;
    var httpRequest = HttpContext.Current.Request;
    //Upload Image
    var postedFile = httpRequest.Files["Image"];
    //Create custom filename
    if (postedFile != null)
    {
        imageName = new String(Path.GetFileNameWithoutExtension(postedFile.FileName).Take(10).ToArray()).Replace(" ", "-");
        imageName = imageName + DateTime.Now.ToString("yymmssfff") + Path.GetExtension(postedFile.FileName);
        var filePath = HttpContext.Current.Server.MapPath("~/Images/" + imageName);
        postedFile.SaveAs(filePath);
    }
}

HTML form

<form #imageForm=ngForm (ngSubmit)="OnSubmit(Image)">

    <img [src]="imageUrl" class="imgArea">
    <div class="image-upload">
        <label for="file-input">
            <img src="upload.jpg" />
        </label>

        <input id="file-input" #Image type="file" (change)="handleFileInput($event.target.files)" />
        <button type="submit" class="btn-large btn-submit" [disabled]="Image.value=='' || !imageForm.valid"><i
                class="material-icons">save</i></button>
    </div>
</form>

TS file to use API

OnSubmit(Image) {
    this.dashboardService.uploadImage(this.componentId, this.fileToUpload).subscribe(
      data => {
        console.log('done');
        Image.value = null;
        this.imageUrl = "/assets/img/logo.png";
      }
    );
  }

Service TS

uploadImage(componentId, image) {
        const formData: FormData = new FormData();
        formData.append('Image', image, image.name);
        formData.append('ComponentId', componentId);
        return this.http.post('/api/dashboard/UploadImage', formData);
    }

Here is a working example for file upload to api:

Step 1: HTML Template (file-upload.component.html)

Define simple input tag of type file. Add a function to (change)-event for handling choosing files.

<div class="form-group">
    <label for="file">Choose File</label>
    <input type="file"
           id="file"
           (change)="handleFileInput($event.target.files)">
</div>

Step 2: Upload Handling in TypeScript (file-upload.component.ts)

Define a default variable for selected file.

fileToUpload: File | null = null;

Create function which you use in (change)-event of your file input tag:

handleFileInput(files: FileList) {
    this.fileToUpload = files.item(0);
}

If you want to handle multifile selection, than you can iterate through this files array.

Now create file upload function by calling you file-upload.service:

uploadFileToActivity() {
    this.fileUploadService.postFile(this.fileToUpload).subscribe(data => {
      // do something, if upload success
      }, error => {
        console.log(error);
      });
  }

Step 3: File-Upload Service (file-upload.service.ts)

By uploading a file via POST-method you should use FormData, because so you can add file to http request.

postFile(fileToUpload: File): Observable<boolean> {
    const endpoint = 'your-destination-url';
    const formData: FormData = new FormData();
    formData.append('fileKey', fileToUpload, fileToUpload.name);
    return this.httpClient
      .post(endpoint, formData, { headers: yourHeadersConfig })
      .map(() => { return true; })
      .catch((e) => this.handleError(e));
}

So, This is very simple working example, which I use everyday in my work.