Fix broken preview images for extension-less external files

[killecaptron]

If no file extension can be extracted from the URL of an external attachment, Part-DB assumed that it is a picture. That is a sensible guess for image URLs, but it also applies to URLs which redirect to the actual file, like the datasheet links of the TrustedParts provider (https://www.trustedparts.com/productredirect?id=...).

Such datasheets were rendered as an in the attachment lists (showing a broken image) and the first one was automatically used as the preview picture of the part by the AttachmentSubmitHandler, so the part page showed a broken main image too.

The attachment type already knows which filetypes it may contain (the "Datasheet" type created by the info provider system is restricted to application/pdf), so use that information for the guess. Attachments whose URL has a known picture extension are unaffected, as are attachment types without a filetype filter.

Existing parts heal themselves: the AttachmentSubmitHandler already removes a master picture attachment which is not a picture anymore, as soon as the part is saved the next time.

[jbtronics]

Thanks for the PR. Seems like a useful change.

I moved the "allowsPicture" logic to the AttachmentType class, as its more a property of them and the logic might be reusable in other places.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.39%. Comparing base (0e1a932) to head (c08fa33).
⚠️ Report is 28 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1520      +/-   ##
============================================
+ Coverage     61.82%   62.39%   +0.57%     
- Complexity     9728     9871     +143     
============================================
  Files           724      735      +11     
  Lines         31302    31745     +443     
============================================
+ Hits          19353    19808     +455     
+ Misses        11949    11937      -12     

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