Go to the Boost admin settings page
Try to add an image (and other file types) using the 'Additional theme preset files' element
Confirm that :
Files with a .scss extension are permitted
Other file types are not.
On 3.1
There is no boost theme preset, so:
Confirm that you can see the 'scss' filetype and that it's mimetype is listed as 'text/x-scss' in 'Admin > Server > Filetypes'
The file manager for the 'Additional theme preset files' allows an image to be uploaded, once saved, the image can be selected from the 'Theme preset' which when saved breaks the style of the entire site.