I think, you could change elsif to a simple if here:

As mentioned by @CheariX, a workaround for now could be to slightly change the code in _layouts/bib.html, starting from line 5:

        <div class="col-sm-2">
            {%- if entry.abbr -%}
            <div class="abbr">
                {%- if site.data.venues[entry.abbr] -%}
                {%- assign venue_style = nil -%}
                {%- if site.data.venues[entry.abbr].color != blank -%}
                {%- assign venue_style = site.data.venues[entry.abbr].color | prepend: 'style="background-color:' | append: '"' -%}
                {%- endif -%}
                <abbr class="badge" {% if venue_style %}{{venue_style}}{% endif %}><a href="{{site.data.venues[entry.abbr].url}}">{{entry.abbr}}</a></abbr>
                {%- else -%}
                <abbr class="badge">{{entry.abbr}}</abbr>
                {%- endif -%}
            </div>
            {%- endif -%}
            {%- if entry.preview -%}
            <div class="preview">
                {% if entry.preview contains '://' -%}
                <img class="preview z-depth-1 rounded" src="{{ entry.preview }}">
                {%- else -%}
                {%- assign entry_path = entry.preview | prepend: '/assets/img/publication_preview/' -%}
                {% include figure.html
                path=entry_path
                class="preview z-depth-1 rounded"
                alt=entry.preview -%}
                {%- endif -%}
            </div>
            {%- endif -%}
        </div>

May not be the cleanest way but seems to work.

Thanks! That worked

I do not understand, why having both, abbr and preview, isn't possible in the current bib.liquid.
If anyone only wants one of them, the bibtex entry can be configured for this purpose.
In contrast, if anyone wants to use both of them, it is not possible (without changing the template).

Since abbr and preview are al-folio specific keys, or at least I don't know if they are used somewhere else, it would be easily controlable via the bibtex entry.

Therefore, I would suggest to allow both side by side in bib.liquid.
If this is okay, I can send a PR.

// cc @george-gca

It is fine for me, but how would this look like? Showing them side by side wouldn't break the layout too much?

My suggestion would be to stack them. I used this on my own website (example).

If you like this stacking, I can send a PR.

I like it, but I think it would be better if the abbr is above the preview. At least I think it would be more consistent, like the conference name always appearing next to the paper title.

I think I once had some reason to put it this way, but I do not rember since it was a long time ago.

I can send a PR for this (with your proposed order, I think this makes more sense).