Calculate sha1 hash of a binary file

[LaundroMat]

I did some testing, and calculating the hash value of a 1Gb file does
take some time using this method.
Would it be wise to calculate the hash value based on say for instance
the first Mb? Is there a much larger chance of collusion this way (I
suppose not). If it's helpful, the files would primarily be media
(video) files.

Thanks,

Mathieu