Ensure valid format

[rdohms]

Currently when outputting a profile that has no specified format, the bundle uses the file's extension, however it does not check if that extension ins actually a valid format, which causes breaks in the backend and broken images.

The bundle should validate the extension against known and compatible image output formats, and fall back to a specific format if it finda nothing valid. (FilterManager)

[makasim]

I think it was fixed in #291

[rdohms]

@makasim maybe write a testcase that can verify this?

[makasim]

we use mimetype\format guesser that comes with symfony. as far as I know it uses file content to guess a mimetype. I think guesser logic is covered in symfony