Allow PSD uploads over 6MB

You can't currently upload PSD files over 6MB. This is because its
mimetype is `image/vnd.adobe.photoshop`. We think this is an image, and
non-GIF images have a limit of 6MB.

To fix this, we could do one of two things:

1. Add a special case for PSD files
2. Increase the size limit for unsupported image types such as this

I went with the second option.

Tested this by:

- adding automated tests
- uploading a ~9 MB PSD file
- uploading a ~1 MB PNG
- uploading a ~27 MB PNG (which worked because it's converted to a smaller JPEG)
- uploading a ~50 MB text file
- trying to upload a ~500 MB text file (which failed, as expected)

Addresses [DESKTOP-1168][].

[DESKTOP-1168]: https://signalmessenger.atlassian.net/browse/DESKTOP-1168

js/modules/types/attachment.js

@@ -1,4 +1,4 @@
-// Copyright 2018-2020 Signal Messenger, LLC
+// Copyright 2018-2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
 const is = require('@sindresorhus/is');
@@ -217,6 +217,7 @@ exports.isImage = AttachmentTS.isImage;
 exports.isVideo = AttachmentTS.isVideo;
 exports.isAudio = AttachmentTS.isAudio;
 exports.isVoiceMessage = AttachmentTS.isVoiceMessage;
+exports.getUploadSizeLimitKb = AttachmentTS.getUploadSizeLimitKb;
 exports.save = AttachmentTS.save;
 
 const THUMBNAIL_SIZE = 150;