Move media type strings to constants and use a content type lookup util

[jessopb]

The code is littered with tests for 'image/gif', or determinetype() or extension for 'type/ext'. Adding a new type support involves editing many files consistently. This should be centralized.

• [ ] Dev can add a file type to publish capabilities by adding a content type to centralized location
• [ ] Dev can add a file type to preview or show by adding a content type to centralized location
• [ ] Dev can choose a viewer by category: IMAGE, TEXT, DOCUMENT, 3D, VIDEO.
• [x] Admin can enable or disable types in config
• [x] Admin can set file size limits with variable granularity.
  • Do we need variable limits for "image" (gif different from jpg)?

[kauffj]

I endorse cleaning this up on the path to filtering and/or as you're adding support for more types.

It seems decently likely that we'll want a file to carry around both it's media type and a higher-level category (like "image", "text" as you say).

If we need content-specific file size limit, I'd guess that category-based would be sufficient. However, if this adds any substantial complexity, I think a single global limit is fine.