HTML input[type=file] accept types

For CSV files (.csv), use:
<input type="file" accept=".csv" />

For Excel Files 97-2003 (.xls), use:
<input type="file" accept="application/vnd.ms-excel" />

For Excel Files 2007+ (.xlsx), use:
<input type="file" accept="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" />

For Text Files (.txt) use:
<input type="file" accept="text/plain" />

For Image Files (.png/.jpg/etc), use:
<input type="file" accept="image/*" />

For HTML Files (.htm,.html), use:
<input type="file" accept="text/html" />

For Video Files (.avi, .mpg, .mpeg, .mp4), use:
<input type="file" accept="video/*" />

For Audio Files (.mp3, .wav, etc), use:
<input type="file" accept="audio/*" />

For PDF Files, use:
<input type="file" accept=".pdf" />