ETD: Thumbnail and Representative should not change when I attach new files

[mark-dce]

STORY As a curator, I do not want the thumbnail or representative of a work to change when I attach additional files, so that my explicit choices about representatives & thumbnails are persistent until I explicitly change them.

ACCEPTANCE

• [x] When I attach an additional file to a work, the thumbnail is not changed
• [x] When I attach an additional file to a work, the representative media is not changed
• [x] When I select multiple files in the file uploader for a new work, the representative and thumbnail are set to the first file in the list on my screen (i.e. the file that appears at the top of the list)
• [x] When I select multiple files in the file uploader for a work with no previous representative and thumbnail, the representative and thumbnail are set to the first file in the list on my screen (i.e. the file that appears at the top of the list)

EXAMPLES 1) A new ETD is created and the user uploads a single JPEG with it. The JPEG should be set as the thumbnail and remain that way even if the user later attaches a PDF 2) A new ETD is created and the user selects a Spreadsheet, a PDF, and another PDF, in that order and uploader them, then the thumbnail should be the first page of the spreadsheet 3) A new ETD is created and the selects PDF4, PDFA, and PDFB in that order, then PDF4 should be the thumbnail and representative media

[no-reply]

Supercedes #86

[jenlindner]

I'm not sure how to test that the thumbnail is the first page of the spreadsheet, but I have a PR almost ready to go that might suffice for the rest, and would love a review to find out if does. It tests adding a pdf to a work with a jpg , and creating a work with an xlsx and two pdfs, and that the representative media settings are preserved in both cases.

[mark-dce]

I don't think we need to test the first page of the spreadsheet - that was me basically saying we should do a quick check that we're not passing any funky parameters to ImageMagick. I think the test is just that the spreadsheet is the thing used to generate the thumbnail - the "first sheet" part of the requirement is overkill.