Cannot delete attachment on newsfeed composer

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### Summary

Can't delete attachment when uploaded and ready to post.

### Steps to reproduce

- Click on the attachment button in the newsfeed composer
- Select an image and wait until it's uploaded
- Click on the trash can icon

### Platform information 

- OS: Pop!_OS 19.10 (Linux)
- Browser: Firefox 70 / Chrome 78

### What is the current *bug* behavior?

Can't delete attachment when uploaded and ready to post.

### What is the expected *correct* behavior?

Should be able to delete attachment.

### Relevant logs and/or screenshots

Console output (Firefox):
```
 TypeError: "this.uploadSubscription is undefined"
    abort https://cdn-assets.minds.com/front/dist/en/main.803bc6d926793668f23b.js:1
    removeAttachment https://cdn-assets.minds.com/front/dist/en/main.803bc6d926793668f23b.js:1
    nG https://cdn-assets.minds.com/front/dist/en/main.803bc6d926793668f23b.js:1
    handleEvent https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    handleEvent https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    Fl https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    _f https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    F https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    r https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    invokeTask https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
    onInvokeTask https://cdn-assets.minds.com/front/dist/en/vendor.779a60b3ce201322a755.js:1
    invokeTask https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
    runTask https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
    invokeTask https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
    _ https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
    m https://cdn-assets.minds.com/front/dist/en/polyfills.055b24b90bf66d29b201.js:1
```

Console output (Chrome):
```
TypeError: Cannot read property 'unsubscribe' of undefined
    at l.abort (main.803bc6d926793668f23b.js:1)
    at l.removeAttachment (main.803bc6d926793668f23b.js:1)
    at Object.handleEvent (main.803bc6d926793668f23b.js:1)
    at Object.handleEvent (vendor.779a60b3ce201322a755.js:1)
    at Object.handleEvent (vendor.779a60b3ce201322a755.js:1)
    at Fl (vendor.779a60b3ce201322a755.js:1)
    at vendor.779a60b3ce201322a755.js:1
    at HTMLDivElement.<anonymous> (vendor.779a60b3ce201322a755.js:1)
    at HTMLDivElement.r (vendor.779a60b3ce201322a755.js:1)
    at e.invokeTask (polyfills.055b24b90bf66d29b201.js:1)
    at Object.onInvokeTask (vendor.779a60b3ce201322a755.js:1)
    at e.invokeTask (polyfills.055b24b90bf66d29b201.js:1)
    at t.runTask (polyfills.055b24b90bf66d29b201.js:1)
    at t.invokeTask [as invoke] (polyfills.055b24b90bf66d29b201.js:1)
    at _ (polyfills.055b24b90bf66d29b201.js:1)
    at HTMLDivElement.m (polyfills.055b24b90bf66d29b201.js:1)
```

### Possible fixes

(If you can, link to the line of code that might be responsible for the problem)

/label ~"T \- Bug" ~"S \- Triage:new" 


Story points: 4.0