fix: check type of imported files

[ZebraVogel94349]

Specifying allowedExtensions for a file picker does only work with specific supported file types (not .sbn and .sbn2). Because of that, the file picker didn't work on Android. Also, on platforms were the file picker worked, you could select any file, not just .sbn, .sbn2 and .pdf files. I changed it so that an error message is shown if you try to import an unsupported file and the file picker works on android now.

[codecov]

Codecov Report

Merging #899 (9ab1883) into main (40d903a) will increase coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 9ab1883 differs from pull request most recent head 2e63037. Consider uploading reports for the commit 2e63037 to get more accurate results

@@           Coverage Diff           @@
##             main     #899   +/-   ##
=======================================
  Coverage   35.88%   35.89%           
=======================================
  Files          94       94           
  Lines        6802     6804    +2     
=======================================
+ Hits         2441     2442    +1     
- Misses       4361     4362    +1     

Files Changed	Coverage Δ
lib/components/home/new_note_button.dart	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

[adil192]

Thanks!