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
