Do not attempt to generate thumbs for svg files (fixes upload of svg files)

[oneiros]

Hi everyone. First of all let me thank you for this fantastic project.

What is this pull request for?

While trying to set up a new alchemy based project a colleague of mine discovered a small problem: He was not able to upload any .svg files. I could reproduce this with current main branch. The exception we hit was:

NoMethodError undefined method `steps' for #<#<Class:0x00007fa2316963f8>:0x00007fa2316d4630> in /app/models/alchemy/picture_thumb/signature.rb:12:in `call'
/app/models/alchemy/picture_thumb.rb:47:in `block in generate_thumbs!'
/app/models/alchemy/picture_thumb.rb:42:in `each'
/app/models/alchemy/picture_thumb.rb:42:in `generate_thumbs!'
/app/models/alchemy/picture.rb:94:in `block in <class:Picture>'

I did a little digging and found that the missing #steps method is called on Alchemy::PictureVariant#image. The documentation of that method explicitly says that it can either return a Dragonfly::Attachment or a Dragonfly::Job. Only the latter seems to have this #steps method.

In this case I think a Dragonfly::Job is only returned for raster image formats for which an actual thumbnail is being generated. For other formats, any conversions are skipped and a Dragonfly::Attachment is being returned.

Notable changes

I was not 100% how to fix this. So please find my first clumsy attempt attached. I decided that the whole thumbnail (pre-)generation probably only makes sense for those formats that can actually be converted this way. So I tried to shut this off as early as possible.

Feel free to discard this if it does not make sense. Also, please let me know if I can improve on this in any way.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[tvdeyen]

Thank you. This makes total sense and is not at all clumsy, but a perfect solution 👍

[tvdeyen]

Rebased with latest main to fix CI issues

[tvdeyen]

Thank you ❤️