File validation

[oliverroick]

Implements #19

• Add audio-length validation, showing an error message when the audio is longer that ~4~5 seconds
• Restrict the allowed file types to flac, mp3 and wav (can be extended)
• Tests that the audio length is below 5 minutes and that the file size does not exceed 1Gb; and displays an error instead of the spectrogram

[oliverroick]

@LanesGood yes correct, my PR description is wrong. I'll have to change the validation a bit today based on Sajjad's input.

[oliverroick]

@LanesGood Added a couple more changes to add the file validation when the file is selected