File previews for common file types

[sfisher]

As part of the suggestions Karthik gave, he wanted to see file previews for common file types. Zenodo also does this for some items. We have (had) something for CSVs which I think maybe dropped out of our code at some point.

I didn't see this in the tickets, so I put this placeholder in case this is something people get to in the future.

Also may require some design thinking.

Related to https://github.com/orgs/CDL-Dryad/projects/1/views/15?pane=issue&itemId=22821524 and it seems like text file downloads are now saved rather than rendered and opened with the system viewer or other viewer which treats them better.

[ahamelers]

It would be reasonable to allow previews for:

• CSV, TSV
• Plain text files, markdown
• Most images
• PDF
• ZIP

I think zenodo also does most of these, with a max of 5MB for preview text and image total

[ahamelers]

I've implemented file previews for csv, tsv, plain text, and small web images.

PDF previews would require something like pdfJS, and zip content lists require us to look again at container_files and our compressed issues. I'll create a separate ticket for these.