Don't generate white preview for txt file

[suse-coder]

Describe the bug

When I create a txt file and write something into it and then save. Then this is shown as preview (basically white image). Even though this is not a bug it looks like one as a normal user does not see this very small text. It would be better to have the normal txt icon instead of a preview displayed.

[JammingBen]

Moved to the Web repository because the Web client requests those thumbnails.

@tbsbdr Do you agree with the desired behavior?

[JammingBen]

Talked with @tbsbdr : We want to skip the preview loading for text (and markdown) files - at least in the file list and the search result previews. It should just show the default text icon we already have. In the right sidebar a preview can make sense, so we would like to keep it there. However, if this appears to be a technical challenge somehow, then we skip is there as well.

[suse-coder]

@JammingBen It is still generating for txt that white preview that looks like a bug:

for image its fine to have a preview, but for txt files it looks strange.