Thumbnails created by the UrlImagePreview plugin are stored as JPEG, regardless which file extension is appended (e.g. picture.png has a JPEG header instead of a PNG header).
To avoid this, we can create thumbnails with the corresponding file format, chosen by file extension. This fixes #234 (closed) , I guess? Also, I set the JPG quality to 95, as 100 does not provide better quality but bigger filesizes.
Additionally, for animated content (GIF) there is only a static thumbnail displaying the first frame.
Resizing of GIF-files with im.thumbnail will create a thumbnail containing the first frame only. Therefore, this commit passes the GIF-file without resizing.
To allow inline animated GIF-files in the chat window, loader.get_animation has to be used in the process of updating the images in the textview.