Attach other documents than images to expenses

[scastiel]

Since #63 it is possible to attach images to expenses. Might be interested to attach other kind of documents (PDF, doc, video…). Uploading the file is not a problem (you can upload anything to S3), but to see the files, an Image tag is used. We’d need to cover other behavior for other files (with a preview, ability to download…).

[CarliJoy]

I would say to keep it simple to only show previews for images and no other file types.

For non image attachment one could only show the filename and a download icon.

[a-camacho]

I made a PR implementing this feature.
Let's see.

#490