Fix blank thumbnails

[alawvt]

Resolve #492.

Thumbnails automatically generated from PDFs produced by 'Microsoft Print to PDF' seem to be a blank thumbnail, e.g. http://hdl.handle.net/10919/77866. (There are other blank thumbnails from several TeX apps, but this issue does not address those.) It appears that ~1% of our ETDs have a blank thumbnail.

To test possible fixes:

I uploaded a replacement thumbnail jpg of the same file name for http://hdl.handle.net/10919/82958.
I uploaded a replacement thumbnail png of the same file name for http://hdl.handle.net/10919/83506.
These thumbnails both appear correctly in Simple Item View. However both display the text "Thumbnail" rather than the thumbnail, itself in the browse page, i.e. https://vtechworks.lib.vt.edu/handle/10919/11041/browse?rpp=100&sort_by=2&type=dateissued&offset=500&etal=-1&order=DESC.

Check later to see if the thumbnails appear correctly in the browse results.

I deleted the blank thumbnail for http://hdl.handle.net/10919/79669 to see if a newly generated one would be better, since some of our filter apps have changed with DSpace 6.

Check this later

[alawvt]

I realized that the filenames of my replacement thumbnails did not include .pdf so I added them. When I did the jpg replacement thumbnail, immediately displayed correctly in the browse results. But the png replacement thumbnail still did not display correctly in the browse results.

It seems like it would be prudent to specify .jpg for manually created thumbnails.

I ran filter-media manually for http://hdl.handle.net/10919/79669 and it created a good thumbnail--yeah! Then I deleted the png replacement thumbnail for http://hdl.handle.net/10919/83506 and ran filter-media manually which created a good thumbnail.

Next I deleted the blank thumbnail for http://hdl.handle.net/10919/81978, which was a PDF produced with http://www.pdf-tools.com and ran filter-media manually. It created a good PDF.

Running filter-media manually on http://hdl.handle.net/10919/81822 replaced a bad thumbnail that had only two lines of text with a thumbnail of the entire title page.

I will go through the theses and dissertation collections and recreate the remaining blank thumbnails (so far have run 35).

[alawvt]

I notice the text on the newer thumbnails is not as bold as the earlier thumbnails (pre-~2016). The new thumbnails are 80px wide. I think the old ones were 100px maximum width, unlimited height. -- addressed in Issue #663.

Some ETDs don't have the cover or title page on page one. These thumbnails will need to manually created. This is continued in #684.