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

ts/types/Attachment.ts

@@ -431,3 +431,13 @@ export const getFileExtension = (
       return attachment.contentType.split('/')[1];
   }
 };
+
+export const getUploadSizeLimitKb = (contentType: MIME.MIMEType): number => {
+  if (MIME.isGif(contentType)) {
+    return 25000;
+  }
+  if (isImageTypeSupported(contentType)) {
+    return 6000;
+  }
+  return 100000;
+};