Don't show "does not appear to be a raster type" warning if the file-type is unknown

[sebastianbenz]

...this can be confusing to users.

[ithinkihaveacat]

Do you happen to remember a URL that triggers this? I tried a few likely suspects but couldn't find anything that has a vector logo. (Also, it looks like the error message has been updated to include examples of the sort of files types that are accepted so maybe that resolves the concern?)

[sebastianbenz]

Not really. I think it was an URL without file extension.