Twitter images don't work (Invalid characters)

[Erisa]

For whatever reason Twitter decided to put :large on the end of some of their image filenames, after the file extension.
This causes the add-on to error when encountering these files, throwing Error: filename must not contain illegal characters
The illegal character in question is the :. Would be good to replace characters like that (Firefox's 'Save as' converts them to spaces) and then find a way to make it so these Twitter images actually have a valid file extension, possibly through the same method as #3

[Erisa]

Was going to keep this open for tracking the resulting problem of .png_large in the filename, but that's just #3 so better to track that problem there instead.
The issue with the colons has been resolved in 2fa79fd .