Добавьте html ваш вклад. Я использую matbutton для замены загрузки по умолчанию.
<div>
<button mat-stroked-button type="button" (click)="filePicker.click()">Pick Image</button>
<input type="file" hidden #filePicker (change)="onImagePicked($event)">
</div>
<div class="image-preview" *ngIf="imagePreview !== '' && imagePreview">
<img [src]="imagePreview" [alt]="form.value.title">
</div>
ts file
form: FormGroup;//in case you are using form
imagePreview: string;
onImagePicked(event: Event){
const file = (event.target as HTMLInputElement).files[0];
this.form.patchValue({image: file});
const reader = new FileReader();
reader.onload = () => {
this.imagePreview = reader.result as string;
};
reader.readAsDataURL(file);
}